User Impersonation with Google Service Account

Hi. I'm trying to implement user impersonation with a google service account and have been having problems for a while. After adding the user to be impersonated as the subject, a token gets created but when calling an API like Google Calendar I get a 401 Invalid Credentials error as if the token that was just created has expired or is invalid.

Do you have any samples of user impersonation? I don't see any in the samples or using Node in Google's documentation. Here is their documentation on the subject: https://developers.google.com/identity/protocols/oauth2/service-account#delegate-domain-wide-authority

Thanks a lot.