How can I retrieve the membership ID?
Not the plan ID but the ID that is shown in Zapier and can be used to create customer profile page slugs in Webflow CMS
Is there a way to query the Memberstack API for past membership IDs?
Hi @XscapeCo 
others may provide a better or more definitive answer, but here is how I solved the first part of your question (video starts at timeline 8:46):
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