XenWord adds several metaboxes to the add post area. One is for the option to create a thread. One is for choosing the forum to create a thread. The final one shows the thread_id if a thread exists. These metaboxes are not present in the WordPress.com editor. This can lead to confusion.
XenWord cannot be active to connect sites. Instead install Jetpack, activate the plugin, then deactivate XenWord. Next, connect Jetpack to WordPress.com then reactivate XenWord.
Keep the option for the thread creation option enabled.
The best approach is to create the text in the WordPress.com editor, select the categories, add tags, the excerpt. Add a featured image. You do not need to press publish.
Log into the site and you will see the new blog entry. Edit the entry and add the forum. Click publish.