Can’t get UserManager from OwinContext in apicontroller

I really misunderstood your question earlier. You are just missing some using statements, I think.

The GetOwinContext().GetUserManager<ApplicationUserManager>() is in Microsoft.AspNet.Identity.Owin.

So try add this part:

using Microsoft.AspNet.Identity.Owin;
using Microsoft.AspNet.Identity; // Maybe this one too

var manager = HttpContext.Current.GetOwinContext().GetUserManager<UserManager<User>>();