Can't do GA tracking across domains when using a form

Hi,
I’m trying to track a clients traffic from e.g. sub.a.com/page1/?id=x to www.b.com/page2?id=x.
So I have added the allowlinker codes on both sides, which should normally add the _ga=y to my destination as www.b.com/page2?id=x&_ga=y.
But it’s not working through forms. Have you come across this before and can you please help me in any direction?
I have also tried to follow this recommendation, but with no luck. I even adjusted the code to identify the form to this:
var myForm = $(‘input[name=“pageVariant”]’).closest(“form”);            addListener(myForm, ‘submit’, decorateForm);
        
            function decorateForm(event) {
              event = event || window.event; // Cross browser hoops.
              var target = event.target || event.srcElement;
            
              if (target && target.action) {
                ga(‘linker:decorate’, target);
              }
            }
        
            // Cross browser way to listen for events.
            function addListener(element, type, callback) {
              if (element.addEventListener) element.addEventListener(type, callback);
              else if (element.attachEvent) element.attachEvent(‘on’ + type, callback);
            }
Best regards
Lars