Form: linking records in a many-to-many relationship

I have created a form to add data to an (custom) entity in Dataverse. As part of this I want to link the record being updated to another record in another entry via a many-to-many relationship (already existing between the two entities).

I have tried adapting the code below as linking records in a lookup (one-to-many) relationship. But this has not worked. Can you advise: is this possible? Should I use another reference?

<form tag with update to the cutom entity>

<input type=”hidden” name=”xxxx_NameOfManyToManyRelationship” value=”{ “LogicalName”: “contact”, “Id”: “00000000-0000-0000-0000-000000000000″ }”>