Unloading hay and grass

[stanleyo]

While trying to unload grass or straw from a self propelled loading wagon, or the Case Jumbo wagon, I get a green box message "Please select the trailer inside the trigger for tipping", in order to sell at the hay barn. What does that mean, and how would I select whatever needs to be changed? Sometimes pressing G lets you offload if it's a trailer, but not with some self-propelled units, like the special Unimog and the Case Jumbo. The unload icon comes up, but when you press 'Q' the warning green box message comes up. Where or how do I select a 'trailer' in the trigger?