Adding services to your DID doc is actually pretty difficult right now unfortunately. 
It requires not only providing your actual PDS password, i.e. app passwords and OAuth logins both don’t work, but it also requires sending you an email confirmation code, before the PDs will allow you to make a change to your DID document.
It’s a pretty steep process, and it’d be nice if there was an easier way to do it.
Having an easy, standardized way to update your DID doc was kind of where I ended up as my desired solution for improving support for “universal login” and off-protocol services.