Android Credential Manager only allows regular apps to handle their own credentials, and not those of other entities.
If your app's users (i.e. end users) need to use credentials that are not associated with your own domain (e.g. if you are a browser app that needs to handle 3rd party credentials), then to be allowed to handle these 3rd party credentials Google Password Manager and Google Wallet require you to be pre-approved for this privileged access.
The following form will open a ticket with Google personnel to review your request.
We make every attempt to get back to you with a response within reasonable time (usually less than 10 business days). We reserve the right to follow-up on this application with any additional questions or requests that will help make a determination in this case, and in those cases the process might take longer.