ULTRA and other radio reports
 

In a campaign you often get radio messages about reported positions of convoys, task forces, and lone ship traffic with a red marker and time of report on the map.

I was wondering how I do that for a created task force? Only thing I can find is the "Contact automatic report (min)" in the properties window, all the ones I see are -1 with 100% probability. Is that the trigger for radio reports? If so, what number should be in there to make a reasonable probability of a message?