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.