Outlook add-in Windows : Button not showing up in the ribbon - outlook

Question

I'm developping an add-in for outlook windows and i'm having trouble setting up the manifest so that the button shows up on the ribbon in the appointment interface.
As of right now, the button that shows up on the ribbon is the button "Office Add-ins" that opens up a window that allow users to select the Add-in i'm developping.
Here is a picture of the button that shows up instead of my add-in icon :
Office Add-ins button showing up
Here is the manifest of my add-in (anonymised)
<?xml version="1.0" encoding="UTF-8"?>
<OfficeApp xmlns="http://schemas.microsoft.com/office/appforoffice/1.1"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:bt="http://schemas.microsoft.com/office/officeappbasictypes/1.0"
xmlns:mailappor="http://schemas.microsoft.com/office/mailappversionoverrides/1.0" xsi:type="MailApp">
<Id>04049d6c-3286-46c1-be8a-965a2a8ee58f</Id>
<Version>1.0.0.0</Version>
<ProviderName>X</ProviderName>
<DefaultLocale>en-US</DefaultLocale>
<DisplayName DefaultValue="Room Finder"/>
<Description DefaultValue="Find a free room for your meeting."/>
<IconUrl DefaultValue="https://someurl.com/assets/logo-64.png"/>
<HighResolutionIconUrl DefaultValue="https://someurl.com/assets/logo-128.png"/>
<SupportUrl DefaultValue="https://someurl.com"/>
<AppDomains>
<AppDomain>https://someurl.com</AppDomain>
</AppDomains>
<Hosts>
<Host Name="Mailbox"/>
</Hosts>
<Requirements>
<Sets>
<Set Name="Mailbox" MinVersion="1.1"/>
</Sets>
</Requirements>
<FormSettings>
<Form xsi:type="ItemEdit">
<DesktopSettings>
<SourceLocation DefaultValue="https://someurl.com/webaddin/taskpane.html"/>
</DesktopSettings>
</Form>
</FormSettings>
<Permissions>ReadWriteMailbox</Permissions>
<Rule xsi:type="RuleCollection" Mode="Or">
<Rule xsi:type="ItemIs" ItemType="Appointment" FormType="Edit"/>
<Rule xsi:type="ItemIs" ItemType="Appointment" FormType="Read" />
<Rule xsi:type="ItemHasKnownEntity" EntityType="MeetingSuggestion" />
</Rule>
<DisableEntityHighlighting>false</DisableEntityHighlighting>
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides" xsi:type="VersionOverridesV1_0">
<VersionOverrides xmlns="http://schemas.microsoft.com/office/mailappversionoverrides/1.1" xsi:type="VersionOverridesV1_1">
<Description resid="residAppDesc" />
<Requirements>
<bt:Sets DefaultMinVersion="1.5">
<bt:Set Name="Mailbox"/>
</bt:Sets>
</Requirements>
<Hosts>
<Host xsi:type="MailHost">
<DesktopFormFactor>
<ExtensionPoint xsi:type="AppointmentOrganizerCommandSurface">
<OfficeTab id="TabDefault">
<Group id="appOrgGroup">
<Label resid="GroupLabel"/>
<Control xsi:type="Button" id="appOrgTaskPaneButton">
<Label resid="appOrgTaskPaneButton.Label"/>
<Supertip>
<Title resid="appOrgTaskPaneButton.Label"/>
<Description resid="appOrgTaskPaneButton.Tooltip"/>
</Supertip>
<Icon>
<bt:Image size="16" resid="Icon.16x16"/>
<bt:Image size="32" resid="Icon.32x32"/>
<bt:Image size="80" resid="Icon.80x80"/>
<bt:Image size="128" resid="Icon.128x128"/>
</Icon>
<Action xsi:type="ShowTaskpane">
<SourceLocation resid="appOrgTaskPaneButton.Url"/>
</Action>
</Control>
</Group>
</OfficeTab>
</ExtensionPoint>
</DesktopFormFactor>
</Host>
</Hosts>
<Resources>
<bt:Images>
<bt:Image id="Icon.16x16" DefaultValue="https://someurl.com/webaddin/assets/logo-16.png"/>
<bt:Image id="Icon.32x32" DefaultValue="https://someurl.com/webaddin/assets/logo-32.png"/>
<bt:Image id="Icon.64x64" DefaultValue="https://someurl.com/webaddin/assets/logo-64.png"/>
<bt:Image id="Icon.80x80" DefaultValue="https://someurl.com/webaddin/assets/logo-80.png"/>
<bt:Image id="Icon.128x128" DefaultValue="https://someurl.com/webaddin/assets/logo-128.png"/>
</bt:Images>
<bt:Urls>
<!-- Separate page for appointments-->
<bt:Url id="appOrgTaskPaneButton.Url" DefaultValue="https://someurl.com/webaddin/taskpane.html"/>
</bt:Urls>
<bt:ShortStrings>
<bt:String id="GroupLabel" DefaultValue="My company"/>
<bt:String id="appOrgTaskPaneButton.Label" DefaultValue="Find Meeting Room"/>
</bt:ShortStrings>
<bt:LongStrings>
<bt:String id="appOrgTaskPaneButton.Tooltip" DefaultValue="Find an available room for this meeting based on your building and floor selection"/>
<bt:String id="residAppDesc" DefaultValue="My company"></bt:String>
</bt:LongStrings>
</Resources>
</VersionOverrides>
</VersionOverrides>
</OfficeApp>
Is there anything wrong with my manifest.xml ? I tried to deviate as less as i could from examples from microsoft teams on Github repos
Edit : It seems that this issue does not happen on outlook 2021
Environment setup : Microsoft Outlook 2016 (16.0.5182.1000) Exchange 2016 (build version 15.1.2375.7) on-premise

Answer 1

In the manifest file a built-in tab was specified:
<OfficeTab id="TabDefault">
Most probably there is no tab on the ribbon with such ID for appointments.
For a custom tab you need to use the following element instead:
<CustomTab id="Contoso Tab">
The OfficeTab is used for built-in ribbon tabs in the following way:
<OfficeTab id="TabData">
If you use the CustomTab element, you can't use the OfficeTab element.
You may check the Find the IDs of controls and control groups section in MSDN if you need to find the built-in tab ID.