Hi,
If GMB and AdWords account has the same login, you could use the same user email address to make a API call. Otherwise, you need to generate access token for your GMB account and can use this access token to make a call by passing this access token, email address, and the busniess account identifier of your GMB account. You could refer code sample in java
here. Samples in other language can be found
here. For more information please refer this location extension
guide. Let me know if you have further questions.
Thanks,
Milind, AdWords API Team.