I’m almost certain that it’s not possible to use DiscourseConnect with Memberstack. For it to work, you would need to be able to add some server side code to your Memberstack site. I don’t think that can be done.
It might be worth asking Memberstack about it. They could implement DiscourseConnect for all their sites in a similar way to what Memberful have done: Integrate Discourse with Memberful | Memberful.com. I’m available for consultation work if they wanted some help with that 