Inserting a new contact via API with custom field that is a date

Inserting a new contact via API with custom field that is a date

I am using the API to add members to my list. I recently created a new custom field that is a date field and now when I try to create a new member it fails with an error 
{
    "code": "2001",
    "message": "Please enter correct XML(or)JSON format",
    "version": "1.1",
    "uri": "/api/v1.1/json/listsubscribe",
    "status": "error"
}

Based on documentation I see the format for date field is mm/dd/yyyy and I am using this.
This is the sample contact info section I am using in the API call: {First Name:Jon,Last Name:Winters,Contact Email:jonwinters@email.com,Last Seen:07/27/2021} where Last Seen is my custom field name.

Any suggestions?

Thanks