I never used Perforce Plugin and I don't know what these checkboxes do. But normally if you want to fix workspace's path you can use Configure->Advanced Project Options->Use custom workspace
checkbox. As described in documentation it does what you need:
Normally you should let Jenkins allocate and clean up workspace directories, but in several situations this is problematic, and in such case, this option lets you specify the workspace location manually. One such situation is where paths are hard-coded and the code needs to be built on a specific location. While there's no doubt that such a build is not ideal, this option allows you to get going in such a situation.
Maybe you have to play also with 'Let Jenkins Create Workspace' and 'Let Jenkins Manage Workspace View' checkboxes.