callbackUrl to dynamic URL like sitename.com/@username not working.

[whoisanonymous]

hey, so i'm trying to navigate the user after they signed in through Google using NextAuth to be redirected to their profile page which has their username in the URL.

Prior to this, i had the redirect set to /profile to make sure everything was set up correctly and it worked perfectly. but now that i want to make the callback URL dynamic with the users' username, but the user becomes undefined when using the code below on the client side on the Google button.

              onClick={() =>
                signIn("google", {
                  callbackUrl: /@${user.username},
                })
              }
            >

so i'm guessing this needs to be done in the callback section of /api/auth/[...nextauth].js? i just have no idea how to do it and i've read lots of NextAuth documentation and still getting lost.

i'm using prisma w/ a postgres db if that helps to know for anyone who knows what i need to do to send the user to sitename.com/@username after logging in from the provider screen.