You can't set inter project allocation. You just get any tasks that are in the hopper and you have selected. HSTB jobs are extremely limited, and it's not a heavily weighted project. So you might get one here or there, but not often. The only way to guarantee you will get only HSTB is to select that project only, and deselect the option of "if there is no work available for the project I have selected, send work from other projects". I discourage that approach though, because if there is no HSTB units available (likely is the case) then your computer will get no work and will eventually go idle.

This lengthy thread goes into the details of why HSTB is limited. If you don't want to participate in some of the projects, you can do that via your My Projects tab if you want all profiles to run the same projects, or from within that same tab you can assign up to 4 different profiles within the device manager.