Machine login as a user

I am connecting our service to IFTTT and as a part of their endpoint testing they want to get userinfo. That is no problem if you are an authenticated user. The problem is that my server needs to send them the access_token they will use for the tests (inluding getting userinfo). How can my server login as a user?

With the normal flow a user needs to type in their credentials and with the M2M flow you are not a user and can therefore not call userinfo.

I found this but it does not work for some reason.