How do we can modify "subject" based on custom filed value (enter by user ) in new request form

Hi Everyone. 
In my request form, I have custom field orderno. 

I want to concatenate the orderno with subject (that will be Ticket's Title).
Currently I have hide the subject field with jquery script and set some default value.

 $('.form-field.string.required.request_subject').hide(); // Hide subject 
 $('#request_subject').val('Insurance new request no:'); // Autofill subject 

What I want to achieve is when user submit the request form the order no which is enter by user should be included in subject.

e.g.

orderno= 12124

subject should be like "Insurance new request no:12124"

Is this possible ? or what can be best alternate for this