OK, I have a fairly untested system implemented for locking propilot flights.
Please do the following.
Dispatch -> propilot -> Locate aircraft
by default this page hunts planes from your pp location, refine selection by aircraft etc
Click the PP lock icon on the right of the row of the aircraft you want
On the next page, click the lock icon next to the destination airport you want.
It will redirect you to the dispatch log if the locking was successful or display the error feedback.
Happy propiloting.
