How to set a Header field on POST a form?

How can I set a custom field in POST header on submit a form?

It cannot be done - AFAIK.

However you may use for example jquery (although you can do it with plain javascript) to serialize the form and send (using AJAX) while adding your custom header.

Look at the jquery serialize which changes an HTML FORM into form-url-encoded values ready for POST.

UPDATE

My suggestion is to include either

• a hidden form element
• a query string parameter

Set a cookie value on the page, and then read it back server side.

You won't be able to set a specific header, but the value will be accessible in the headers section and not the content body.