I've figured out TuxGuitar for the most part, and although I can do simple repeats, I can't find where "D.S. al Coda", "To Coda", and the symbol buttons are.
How do I enter Segno, Coda, D.S. al Coda into TuxGuitar?
Unfortunately, TuxGuitar does not currently have this capability. Posts on their forum have been suggesting such an implementation for years, but it has not yet been acted upon.
If you need software that can provide such markings, I recommend free/open-source programs like MuseScore or LilyPond (my personal favorite). Finale, Sibelius, and Dorico also have these capabilities, but you'll need to pay for the software.