You have a few different options for getting your Extension out to MainWP users. With any of these options just send us an email at support@mainwp.com and we’ll be sure to mention your Extension in our weekly email.
Free Extensions
If you plan on making your Extension free for anyone to use you can add it directly to WordPress.org. If you have never added a plugin to WordPress.org before there is a quick tutorial on WPBeginner
Paid Extensions
If you plan on selling your Extension you can:
Add it to your own site and promote (harder)
Add it to http://codecanyon.net (easier)