SEPM 12.x Custom Scan Config

Greetings - I have some scan exclusions set up in SEPM for some of our servers. However, we wish to scan these locations once a week. When I try to set up a custom scan in SEPM, I do not seem to be able to enter a drive letter and path. It appears that the only options available are some preconfigured variables. None of these are  suitable for what I want to scan once a week. Can anyone shed light on this issue for me?

Thanks!