Gmail API for managing multiple signatures - google-api

Question

Google recently released an update to Gmail to bring support for multiple signatures. Ref: https://support.google.com/mail/answer/8395.
I do not see anything in the API documentation at https://developers.google.com/gmail/api/v1/reference that talks about how to manage those multiple signatures. How can I:
create a new signature
update a specific existing signature
associate a signature to an email address - both the "for new emails use" and "on reply/forward use"
Is there any documentation on this?

Answer 1

It is a bit hidden, the signature(s) need to be created with Users.settings.sendAs: create or update
As specified for the resource, signature is one of the parameters that can be modified and you do not need to create a new alias, but can also apply this method to your primaary email:
Settings associated with a send-as alias, which can be either the
primary login address associated with the account or a custom "from"
address.
Important: Access restricted to service accounts that have been delegated
Sample with Apps Script:
function createAlias() {
var alias = 'your primary email';
var signature = 'Your signature';
var service = getOAuthService();
service.reset();
if (service.hasAccess()) {
var url = 'https://www.googleapis.com/gmail/v1/users/me/settings/sendAs'
var headers ={
"Authorization": 'Bearer ' + service.getAccessToken(),
"Accept":"application/json",
"Content-Type":"application/json",
};
var resource ={
sendAsEmail: alias,
signature: signature,
};
var options = {
'headers': headers,
'method': 'POST',
'payload':JSON.stringify(resource),
'muteHttpExceptions': true
};
var response = UrlFetchApp.fetch(url, options);
Logger.log(response.getContentText());
}
}
Necessary scope:
https://www.googleapis.com/auth/gmail.settings.sharing