As Georg said, there is currently no way for a layout application to automatically apply justification features. Putting things in the JSTF table is one option, and in theory it means that your font will have justification alternates enabled whenever applications do get around to supporting it - but I am not convinced that they will, because there are various problems with the JSTF specification anyway.
Depending on what you want to do, another option would be to release the font as a variable font with the expanded glyphs available via the GEXT (glyph extension) axis. You can see some examples of GEXT axis fonts here, and I have implemented justification via GEXT axis for Armenian fonts in the web browser using my newbreak justification engine. (Because the Armenian font was not a free one I don’t have an example of that online, but it does work - see the Arabic demo for a similar concept which uses the width axis rather than GEXT.)