Adding Flags to Metadata using Zendesk API

Hi there. So I'm creating Zendesk tickets using the API in PHP, and I want to add flags:[3] to the metadata so that it shows that the ticket requester was not signed in when the ticket was created. However, I have yet to find a way to edit the metadata in this way; all of the ways that I've found to edit the metadata when creating a ticket have just added it to the 'custom' field, as opposed to actually adding this flag. Was wondering how I would go about doing this? Thanks!