micrsoft back up

Trying to schedule automatic backups for a domain user. When setting up the task you are asked for a runas user and password. To get it to work I entered user’s net password. The problem is, when the password expires and is changed, the task will no longer run unless the password within the task is changed to match.

I’ve tried entering a domain user’s name (with admin rights) in the runas line, but the setting never seems to hold. It always reverts back to the current user’s name once you click OK.

Thanks