There is currently no JS Helper functions to help support that.
And off hand I’m not sure what the ruling would be for Twitch as from your iframe/extension you’ve have to whitelist twitch.tv and you’d be essentially “offsite” linking to https://twitch.tv/STREAMER/subscribe which would be a suboptimal payment flow.
It’s not “against the rules” but I don’t think Twitch would accept an extension supporting that, but should be a thing Twitch could add to the JSHelper to like they do with the follow button.