Due to how the integration with Meta works, posting to Facebook Groups is finished via mobile notifications. To receive them, make sure you download the Postcron app to your phone, you can find it here for Android and here for iOS, and log into your Postcron account on the app.
Scheduling
Scheduling to Facebook Groups works the same way as other social accounts. First, make sure you connect your accounts to Postcron. Then, select the accounts you will be working with on the top part of the screen, and click on the Composer to create your post.
Schedule your posts using the available options: Schedule to manually pick the date and time, Predefined Times, or Post Now. (Keep in mind that Post Now is a Premium feature on Postcron.)
Posting
Once it's time to post, Postcron will send you a push notification. If you use multiple Facebook profiles on your phone, before tapping the notification, make sure you're logged into the correct Facebook profile.
Then, open the notification and check your post, then click on Continue on Facebook and look for Facebook amongst your phone's sharing options to be redirected to the Facebook app.
Now, paste your text or link onto Facebook's post creator and edit your post if you need. You can add tags, location, and any rich content available on Facebook. Once your post is ready to go, simply click on Post.