I'm the first to admit as I do not send messages often, it fools me each time how to start one , it's not easy!
When the large white box appears it would seem you write your message there, you don't, below in faded writing is send message, click on that and write the message there, then when finished, a big blue send message tab appears click on that !
I use a laptop to do the above , I don't use the app where instructions may be different there.🤔
@perfectjointer