POST to admin exclusive endpoints in production

We have some endpoints for admin-exclusive use. In our app.yaml we have something like this:

handlers:
- url: /_ah/api/appname/v1/admin/.*
  script: main.api
  login: admin

Usually I use Postman when I need to POST a special request, but for requests to login: admin endpoints, Postman redirects to and opens body of the Google Login page, which doesn't actually allow me to login.

If I open such redirect URL in the browser, I'm actually able to login, but after that, I'm redirected to the GET version of the endpoint, which doesn't help as well. I don't think the browser has a feature which easily allows me to make a custom POST requests after I've login.

So in the end I see two alternatives... one would be "importing" the authentication cookies into POSTMAN after login. The other would be use javascript in the browser's console to "manually" make the POST request I'm trying to make.

Both options seem to me somewhat overcomplicated, and I feel like I gotta be missing some easier way of doing what I'm trying to do.

So my question is what's an easy way to make a POST request to a deployed gooogle cloud endpoint which is protected by login: admin?