Actually, I have already added this feature for the upcoming update. :)
I still want to test the update more before publishing it though.
If you don’t want to wait, you can do it “manually” like this:
if (!textbox_exists()) {
textbox_create("Hi1.","Hi2.");
// call textbox_set here and set a character name with it
}else if (textbox.message_current == 0) { // Textbox exists and current message is 0 (you can try different message numbers, I'm not sure which number is considered first etc.)
// call textbox_set here and use another character name
}