Add site-specific scriptlet

Related issue:
uBlockOrigin/uAssets#19903

assets/resources/scriptlets.js

@@ -3452,6 +3452,29 @@ function preventCanvas(
     });
 }
 
+/******************************************************************************/
+
+builtinScriptlets.push({
+    name: 'multiup.js',
+    fn: multiup,
+    world: 'ISOLATED',
+});
+function multiup() {
+    const handler = ev => {
+        const target = ev.target;
+        if ( target.matches('button[link]') === false ) { return; }
+        const ancestor = target.closest('form');
+        if ( ancestor === null ) { return; }
+        if ( ancestor !== target.parentElement ) { return; }
+        const link = (target.getAttribute('link') || '').trim();
+        if ( link === '' ) { return; }
+        ev.preventDefault();
+        ev.stopPropagation();
+        document.location.href = link;
+    };
+    document.addEventListener('click', handler, { capture: true });
+}
+
 
 /*******************************************************************************
  *