Once an admin has generated a code for you, you can log into the Nextcloud web interface with your usual login name and password. If you are prompted with a selection of two-factor auth providers, choose the Admin code provider.
You should have been redirected to a page asking for your admin code. Now it's time to enter or paste the code in the input and submit it. If the code is valid and has not expired yet, you'll be logged in after this step. In any other case, please contact your admin again for assistance.
Note: since the code can only be used once, you'll have to make sure that your two-factor setup is up to date. Otherwise you'll be unable to log in again.