Which REST API should be used to add a user to a role?

That is a confusing one. You can add a user to a role by "Creating" a role with Include="user" and the user name or ID via:

But is it the same call if the role already exists?  It is just poorly documented or missing a decent example?  Am I over-thinking things?

But is it the same call if the role already exists?  It is just poorly documented or missing a decent example?  Am I over-thinking things?

The same REST endpoint is used for creating a role and creating a role-user relationship. The differentiation is via the Include="user" option. I agree this is confusing.

KMSigma said:

But is it the same call if the role already exists? 

Yes.