You would just stuff the value of the Related Project with the hard-coded Record ID# of that dummy project so that nobody needed to fill it in or even think about it.
If these types of tasks are much simpler than the normal project tasks in terms of what other fields are necessary and relevant, you could even set up a different form (clone the original) and then remove what's not necessary, and then part of your button formula would be to specify the formid to load that slimmed-down personal task form instead of the regular one.