make sure prefix is not empty before submitting

app/dashboard/templates/dashboard/custom_alias.html

@@ -28,6 +28,7 @@
       <div class="row mb-2">
         <div class="col-sm-6 mb-1 p-1" style="min-width: 4em">
           <input name="prefix" class="form-control"
+                 id="prefix"
                  type="text"
                  pattern="[0-9a-z-_]{1,}"
                  title="Only lowercase letter, number, dash (-), underscore (_) can be used in alias prefix."
@@ -95,12 +96,18 @@
     $("#submit").on("click", async function () {
       let that = $(this);
       let mailbox_ids = $(`#mailboxes`).val();
+      let prefix = $('#prefix').val();
 
       if (mailbox_ids.length == 0) {
         toastr.error("You must select at least a mailbox", "Error");
         return;
       }
 
+      if (!prefix) {
+        toastr.error("Alias cannot be empty", "Error");
+        return;
+      }
+
       that.closest("form").submit();
 
     })