Posting String Collections in FormData

[jamsoft]

I'm having trouble figuring out how to get this working:

I have the following endpoint configured:

[Post("/create")]
Task<ApiResponse<EntityIdDto>> Create([Body(BodySerializationMethod.UrlEncoded)] Dictionary<string, object> data);

And in my DTO object I have defined a method to build the dictionary:

public Dictionary<string, object> ToFormDictionary()
{
}

Two of the properties on this are collections of strings IList<string>. I'm having issues getting the external API to accept the values:

{ "race[]", string.Join(",", Race) },
{ "ethnicity[]", string.Join(",", Ethnicity) },

The only documentation I have is a postman collection, and for this call the body type was set to form-data. Helpfully, the detailed example call in the postman collection fails with a stack of validation errors, so I don't even have an example at the moment.