You cannot send messages through the mobile game app.
You can only send the INITIAL messages through the WGT website. And then can send messages with the WGT site through your “my messages” tab
You CAN send the INITIAL message using your mobile phone by accessing the WGT site through a browser that has the ability to play “Flash” content. Most of them do not do that though. You do not need “Flash” to send messages on the WGT site once the initial one is sent or if someone else sends you the initial message
There is a FREE one that works for now called “Puffin Web Browser” in the App Store but it seems it will only function until Oct 2019. There is also a pay version of it ($6-7) but I’m not sure how long it will work.