Have your reinforcements at a waypoint with the condition field set to: Reinforce
The next waypoint should be where you want them to go once they have been called for.
Have a radio trigger that has the on activation field set to: Reinforce = true
For the avoidance of doubt: There is nothing special about the word Reinforce. You could use xxyyzz so long as you use the same thing in both the waypoint and the trigger.