post https://{region}.mixpanel.com/engage#profile-numerical-add
Takes a JSON object containing keys and numerical values. $add will increment the value of a user profile property. When processed, the property values are added to the existing values of the properties on the profile. If the property is not present on the profile, the value will be added to 0. It is possible to decrement by calling "$add" with negative values. This is useful for maintaining the values of properties like "Number of Logins" or "Files Uploaded".