Subscribers only (HOW)?

So I am reading the help stuff and I came across this.

Subscribers-only mode #

In a box below the above text is says, subscribers_only: true

Then it goes on to say “You may want to ensure that only logged-in user can submit your contact form. In Such Cases, use the subscribers-only mode. In this mode, non-logged in users can’s submit the contact for and will see a message informing them that login is required, while logged-in users can use ti as usual

GREAT!! HOW? I have tried copy and pasting the text into the “Additional Settings” box and that did not work. Beyond that I have no idea how to make this work. I have a page were my members can submit photos but I do not want a bunch of random photos. Only from members but the tutorial text gives me very little to go on as to HOW to do this.

Thank you for any pointers

Jeff

The page I need help with: [log in to see the link]