Asp.Net Core Identity - Using a stored procedure to save new users

You should keep using _userManager.CreateAsync, but configure it in such a way that your own implementation of a UserStore is used.

In Startup.cs:

public void ConfigureServices(IServiceCollection services)
{
    services
        .AddIdentity<MyUser>()
        .AddUserStore<MyUserStore>();
}

Then, create a custom UserStore, where you can call your context class (including calls to stored procedures):

public class MyUserStore : IUserStore<MyUser>
{
    private MyContext _db;
    public MyUserStore(MyContext db)
    {
        _db = db;
    }

    public async Task<IdentityResult> CreateAsync(MyUser user, CancellationToken cancellationToken)
    {
        // simply store entity in DB
        user = _db.Users.Add(user).Entity;
        await _db.SaveChangesAsync();

        // or run stored procedure
        await _db.Database.ExecuteSqlCommandAsync("usp_MyUserCreationStoredProcedure @p0, @p1", user.FirstName, user.LastName);

        return IdentityResult.Success;
    }
}

You can check the relevant documentation on how to create custom user stores for more information.