When their is a contact custom attribute with a field type of Select and Allow Multiple - Yes, set multiple options for the field via the api. For example, if I had the contact custom attribute: Multiple Select Test with options: Option 1, Option 2, Option 3, Option 4 and I want to select Option 1, Option 2 and Option 4 Use the Update Contact endpoint like this: { "contact": { "custom_attributes": { "multi_select_test": ["Option 1", "Option 2", "Option 4"] } } } Or even better, make select custom attributes work like Tags. Allowing us to retrieve all the possible options for a specific custom attribute and use the option id or ids to set the field's value.