That depends on your theme. I suggest taking a look at how it works when using one of the default WordPress themes, like Twenty Fourteen. Normally, comments for submitted posts work just like comments with regular posts.
After I mannually allowed comments, the reply field appeared.
How can I allow comments automatically?
Visit Settings > Discussion > “Default article settings” > “Allow people to post comments on new articles”, also have a look at the other comments settings, and read through:
http://codex.ww.wp.xz.cn/Comments_in_WordPress
Thank you Jeff,
In the setting tab, “Allow people to post comments on new articles” is already checked, but that still doesn’t enable commenting.
Here is what I did to make commenting possible by dafault: In the file user-submitted-posts.php, add
$postData['comment_status'] = 'open';
before
$newPost = wp_insert_post($postData);
Thanks for posting, huynhlv54. Comments should be enabled by default without any need for such code, but I will take a look for the next update and see what’s up.