Using the link:
tg://resolve?domain=username1
It shows a popup to message the person in telegram, but is there a parameter to also attach a message to this handler? There isn't really any documentation for this it seems.
As descibed in my previous answer, there is almost nothing to find about the Telegram URI Scheme, except for some feeds were people complain about the fact it has been broken since an update;
This answer that says tg://msg?text=your MsG!"
should work on IOS only. There is an comment below saying it also works on android
Twitter conversation, Telegram says msg?
should work, but the'll have something better soon
?msg=
.Unforunaly Telegram stopped responding on the twitter conversation, until now there is no real documentation on this.