You will be prompted for an optional secure token for macOS 10.13.x or later. Enter the admin account username and password you created earlier to enable the secureToken for the mobile account.
If FileVault is enabled, you'll need to run the following as well to properly enable FileVault for the new account after the secure token prompt:
sudo fdesetup add -usertoadd <nsid>
When prompted, enter in the credentials for an account that can already unlock the disk. You'll be prompted for the password for the <nsid> account after.