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.
I've found the solution to this. Though it's been 4 years since this question was asked, and remained unanswered yet. Probably, you might have found it yourself now.
Anyways, I'm going to share what solution worked for me: by adding a text param in your URI
E.g.: tg://resolve?domain=username&text=your+msg+here
Make sure to use a +
replacing any white spaces in your text param.
I haven't tested it on IOS yet, but works good as expected on Android devices.