It is in the menu in both Scriv2 and Scriv3 so you can bind it to a keypress using systen preferences, just use ⌘⇧? and type line to find its name (there are three line options).
Sorry the ⌘⇧? is only to search in the Scrivener menu’s to find the specific menu name you need (it must be an iten not a parent menu name). THEN you take that name (here we choose Page-Spanning Line) and use it to create a setting in System Prefs:
Appendix §7 of the user manual, though it includes descriptions of what commands do so isn’t really a simple “cheat sheet” style. There are some cheat sheets for Scrivener (google for them), or you can use software like KeyCue which generates a cheat sheet on the fly for any app: