I have no idea how to send a message on the app or mobile browser, as I do not use those glitchy devices.
On a desktop I admit it's not that obvious either !
I'm on a laptop using Chrome, when you reach the Messaging page, bottom right click on Send Message, yep you haven't written it yet , said it wasn't a clear method!
Then again click on the flashing I symbol next to Send message, and it's there, that you type your message. When finished click on the upward tilted arrow.
I always click the box send to my email , then I know it's definitely been sent.
@trevora9773