First of all, you need to get your Calendly embed code, as described here. Further on, create a corresponding field in your data source that you're using for the List Details block. In the example shown below, we've created a Single line text field named "Calendly" in Airtable and added the embed code to one of the rows.

Next, we’re going to add an Embed type field to the List Details block and link the "Calendly" field from Airtable.

That's it. Now the Calendly embed will start appearing on the list item as shown below.

You can embed other publicly accessible integrations this way as well. Some ones we’ve seen aside from Calendly are Trello cards, Google Docs, YouTube videos, and Github issues.