A texting app has to have a companion app on the watch to be able to initiate a text. I think it would be up to the developers of Textra to make a companion app. Google messages has a companion app.
I use Textra and chomp and they both work on the watch, I get messages all the time and I can respond on the watch and it always goes through, I think you just have to enable it in the app settings on the wearable app
The OP was asking about initiating a text, not responding to one. You can only initiate texts with Google Messages or Samsung Messages, both of which have companion apps installed on the phone.
The good news is you can use either Samsung or Google messages on the watch to initiate a message and it will stiil appear in your Textra app on your phone. You may get duplicate notifications, though, unless you turn off notifications for the other messaging apps on the watch.
