My understanding is that the MultiSelect Option set type field is fairly new. It does not appear as an option when I create a new Marketing form field - so I cannot map anything to it. This needs to be supported.
Just saw this is a duplicate and I don't have the option to delete it.
Category: Marketing Pages, Forms, and Surveys
Thank you for your feedback!
This has been released as a part of the Multiselect Option fields functionality in February 2020.
D365 Marketing Product team