Hi @Anonymous User
If you haven't purchased the movie pass yet, then the credit on your account should be automatically offset when you buy the movie pass.
Should you have a rolling movie pass subscription, then the credit will be used against your next payment (assuming you haven't got another pass where the due payment date is before the movie pass where the credit will be used on the first due payment pass first and will continue on reoccurring next due payment dates across all passes until the credit runs out).
Once the credit is added to your account it will automatically keep offsetting any rolling payments until the credit runs out.
Maybe this link NowTV article will help understanding how the Tesco Clubcard credit works ?