I'm also surprised to Google and find that this isn't a thing, any updates since March?
Users can sign up to my SaaS which has multiple plans. User registers, then is asked to select a plan. All plans start with 10 day free trials, so the 'Start free trial' button triggering a 'Pay Now' dialog is misleading. I have to set the description of the modal to be something like 'You will not be charged until your free trial has elapsed', but it gets truncated so you have to shorten it.
It'd be much easier if I could have a 'Subscribe' button, or perhaps a flag that switches it to a Subscriber dialog with a built-in notification about the fact that you won't be charged yet.