How to use !FindInMap in !Sub | userdata section of Cloudformation - yaml

Question

How can I use !FindInMap in userdata section.
In the following userdata I want to update the MainSshKey with the mapping data.
Mappings:
AccountToStage:
"123456789012":
StageName: Beta
Beta:
us-east-1:
MainSshKey: ssh-rsa AAAAB3NzaC
AdminSshKey: ssh-rsa AAAAB3NzaC1
userdata:
Fn::Base64: !Sub |
#cloud-config
users:
- name: main
ssh_authorized_keys:
- ${MainSshKey}
- name: admin
ssh_authorized_keys:
- ${AdminSshKey}
this is what I have tried
#cloud-config
users:
- name: main
ssh_authorized_keys:
- ${MainSshKey}
- MainSshKey: !FindInMap
- !FindInMap
- AccountToStage
- !Ref "AWS::AccountId"
- StageName
- !Ref "AWS::Region"
- MainSshKey
- name: admin
ssh_authorized_keys:
- ${AdminSshKey}
Cloudformation is not able to resolve this.
Note: If I defined MainSshKey as a parameter, and it works fine, doesn't seems to work using FindInMap
Any pointers are much appreciated

Answer 1

You have to use a list form of Sub. So your could should be something along these lines. Note that you will probably fix all the indentations and further change it to work. Nevertheless, the list form of Sub is the key to your issue.
Fn::Base64: !Sub
- |
#cloud-config
users:
- name: main
ssh_authorized_keys:
- ${SubMainSshKey}
- name: admin
ssh_authorized_keys:
- ${AdminSshKey}
- SubMainSshKey: !FindInMap
- !FindInMap
- AccountToStage
- !Ref "AWS::AccountId"
- StageName
- !Ref "AWS::Region"
- MainSshKey
ALso, this is incorrect userdata, so I'm not sure what do you want to achieve.

Answer 2

I was able to solve this using the following, the plus tells it to keep a newline afterwards. Arg1 tells it to substitute the result of the Fn::FindInMap for the MainSshKey in arg0.
Fn::Base64: !Sub
- |+
#cloud-config
users:
- name: main
ssh_authorized_keys:
- ${MainSshKey}
- name: admin
ssh_authorized_keys:
- ${AdminSshKey}
- MainSshKey:
Fn::FindInMap:
- !FindInMap
- AccountToStage
- !Ref "AWS::AccountId"
- StageName
- !Ref "AWS::Region"
- MainSshKey
Also if you require substituting two values (example: MainSshKey and AdminSshKey), you can use the same list Sub construct example below.
Fn::Base64: !Sub
- |+
#cloud-config
users:
- name: main
ssh_authorized_keys:
- ${MainSshKey}
- name: admin
ssh_authorized_keys:
- ${AdminSshKey}
- MainSshKey:
Fn::FindInMap:
- !FindInMap
- AccountToStage
- !Ref "AWS::AccountId"
- StageName
- !Ref "AWS::Region"
- MainSshKey
AdminSshKey:
Fn::FindInMap:
- !FindInMap
- AccountToStage
- !Ref "AWS::AccountId"
- StageName
- !Ref "AWS::Region"
- AdminSshKey