use this.assign

assets/javascripts/discourse-assign/controllers/assign-user.js

@@ -72,8 +72,8 @@ export default Controller.extend(ModalFunctionality, {
   },
 
   @action
-  handleTextAreaKeydown(value) {
-    if ((value.ctrlKey || value.metaKey) && value.key === "Enter") {
+  handleTextAreaKeydown(event) {
+    if ((event.ctrlKey || event.metaKey) && event.key === "Enter") {
       this.assign();
     }
   },

assets/javascripts/discourse/templates/modal/assign-user.hbs

@@ -67,7 +67,7 @@
       "discourse_assign.assign_modal.assign"
     }}
     @icon={{inviteIcon}}
-    @action={{(action "assign")}}
+    @action={{this.assign}}
     class="btn-primary"
     @disabled={{disabled}}
   />