Question: How can I add some of the infinite length ligatures likes the ones on here to my own font?

[iCraft85]

Just wanted to know how I can infinite length ligatures to my own font? because I want to make ------ and _____ infinite length ligatures so that they don't have space in between them but I don't know how to begin making ligatures.

[tonsky]

You can look here for the code that does underscore in Fira Code https://github.com/tonsky/FiraCode/blob/master/features/calt/underscores.fea