Service Request Items for Unknown external requesters

Is there a way to open visibility of a service request item to external users, the way that the Incident form is? Currently, the visibility status of "All Requesters" only extends the visibility to existing requesters. But how can we enable a form for future unknown external users?  

At the moment we are paying for an alternate form service for these type of users to fill out a service request form and it is emailed as an attachment to our FreshService environment. But, this is not ideal, as it then becomes multiple locations to manage forms and workflows.