Sometimes when administering a Dynamics instance, I’d get a spreadsheet of contacts or accounts and be asked to add them to a marketing list. Not the simplest thing it turns out, so a quick Flow using a HTTP action to make it happen:
You need two GUIDs, the contact GUID (or account / lead) and the marketing list GUID, which you can get from the list form URL. In my use case, I actually built a little canvas app that the users could run and they would enter the marketing list GUID there, and an Excel spreadsheet in Teams that contained the contacts (by the nice contact ID), and this would execute the Flow.
So, the action to make the marketing list populate is below, looping through all the contact list.
The body is:
A word of warning, not the fastest to run, so turn concurrency up, and not sure it’s overly useful if the marketing list is in the thousands …