Self-Service form channel to Web

Self-Service form channel to Web

Hi,

When our customers fill in the self-service form in Desk portal, all new tickets are registered on "Web" in the "Channel" field.
Customers cannot change the "Channel" field.
We've created the value "Self-Service Portal" in the "Channel" field a long time ago and that one is set to default when someone submits a ticket but it's still going to "Web" after submit.

How can we change this that it goes to "Self-Service Portal" by default when a customer submits a ticket?