Swashes in script font

How does one create, say, 1-10 end swashes within a font so that the user can type something like “_1” or “_2” after the last letter and add the swash?

That would be ligatures. Read a bit about openType feature. And to automate this, you make some glyphs like e_underscore_one and then Update the features in Font Info > Features. https://glyphsapp.com/tutorials/ligatures

Thanks for the reply. I’m assuming the Font Info>Features action is only available in the full Glyphs, not Glyphs Mini? I’ve created a .swsh glyph for the “e” but when using the OT panel in Illustrator the swash button is greyed out because I can’t find the “update” selection.

Correct. .swsh is not supported by Mini. A list of supported features is in the appendix of the Mini handbook.