Can not abort sending a personal message to another rider in companion app on iPhone.
The only way is to send a faux message (like a period) and then leave the message sending screen as keyboard occludes the bottom toolbar.
On the iPhone keyboard on the ZCA you should be able to drop the keyboard and then click on one of the menus on the bottom. I have done this a lot over the years.
How do you drop the keyboard?
I may be doing a ride later and can check, but I think it is on the left-hand side.