Although you'd think the Extra tube option would be helpful with this, unfortunately, extra tubes can only be used to connect existing tubes that are already in place. So the seat stays would still need to be connected to the seat tube before an extra tube could be added to it.

My first thought in looking at your example was that this looks a lot like a wishbone stay without the top section of the wishbone. Unfortunately, the wishbone option doesn't break things down into segments with detailed info about how those joints get mitered. Instead, it gives you the basic lengths of the segments and gives you the creative license to miter as you see fit.

Another approach which admittedly also doesn't treat these elements as unique segments is to add a really sharp bend at the top of the stay to represent the point where these two segments connect.

I've created an example below. I'm sorry this isn't a perfect match for your design, but might yield some useful info for mitering the top segment to the seat tube.

You can export this design to FreeCAD and do further modifications there to better represent your joint.