In reality at today it seems we can have a mix in at least two different ways, in the kind of:

either tg://resolve?domain=username&text=hello+there

or https://t.me/username?text=hello+again

that will open on the telegram desktop the chat to the given user with the text message compiled, but it still needs the manual entry of the send message bar by pressing enter button. Is there any way to escape the need for this, by passing some other specific token to the browser address or some settings on the browser or telegram desktop app?