How to put a custom message in the message module template?
Checkout the setArguments(), getText() and the processArguments() function in the Message Entity class:

$message = Message::create(['template' =&gt; 'sfbu_progress']);
$message-&gt;setArguments([
   '@my_argument' =&gt; 'my_argument_value',
]);
$message-&gt;save();


And in your Template which you create over the UI write the @my_argument to the textarea. It should be replaced with your text.

It is also possible to add an callback function as argument. Therefor checkout the declaration of the argument property in MessageTemplate Entity class.