Actually, while it might be considered to be a bug in the Text Image Generator panel (that it doesn’t deal properly with text that you enter using Option+Enter to include returns), you can still try this manually in your script like this:
Click (Text:"This is" & return & "some text.")
There is still no guarantee that this will work for you. Fundamentally, the TIG system doesn’t have any special support for multi-line text at this time.
Another approach you might consider (depending on exactly what you need to do) is to look for each line of text individually, something like this:
put ImageLocation(Text:"This is") into firstLineLoc
Click (Text:"some text.", SearchRectangle:((0, firstLineLoc.y), RemoteScreenSize()))
This example uses a SearchRectangle to ensure that the second line appears somewhere below the first line (although it could be anywhere in the lower “half” of the screen). With a little work you could construct a search rectangle that would limit the search to a smaller area directly below the first line. There are some useful SearchRectangle example functions here that may give you ideas on how to go about this.
You may also need to get the location of the second line and use that to calculate some other location, rather than simply clicking on the second line as my simple example does.