Add a Facebook account using the Personal App method.

The App method is the best method for adding a Facebook account. It is safer, more relevant, and works 100% of the time: no errors, no chance of being blocked by Facebook. You can get permission to share posts on your managed groups and own pages. Facebook does not officially allow users to share posts on member groups, and account using the Facebook Apps. Read more about the announcement.
Firstly, you should create your personal Facebook App, and send it to the review. It may take some days to be approved. After approval, you can add your account using your personal App.
For creating personal Facebook App, and sending it to the review, please follow these steps:

Go to Facebook Apps
Click the Add a New App button;
Select the "For Everyone Else"

Type your App name. Try to have a different name to notice it easily among all Facebook Apps. Click the Create App ID button;
Click the Set Up button within the Facebook Login section;

Select the WEB, type your Site URL, and click the Save button.

Go to the Facebook Login > Settings menu and type "Valid OAuth Redirect URL" and save settings;

You can copy your redirect URL on the Apps tab of the Plugin:
Go to the Settings > Basic and fill the form. You need to type your website and your information.

Go to the App Review > Permission and Features menu;
Find the pages_manage_posts, pages_read_engagement, publish_to_groups, Groups API permissions, and click the Request button in front of each one.

You need to have the manage_pages, publish_pages, publish_to_groups, Groups API permissions for the lower version of Facebook API (<7.0).

Click the "Continue the Request" button. Fill the form for each permission, and send your App to review. You need to wait for up to 5 days. A random Facebook worker will review it. The worker needs to test it on your website. That is why you need to create a user for the App tester and add the App to your website so that they can test it. If that worker does not have knowledge about plugins or WordPress, he does not review your request properly and declines it. Please try to re-submit the same request again. Eventually, your request will be accepted. You can also ask your questions to the App testers;

Here is a sample detail for the review form. Add your website URLs and change screenshots to your own App:

Website: "type the URL of your website"
Testing login: "type a username of your website"
Testing password: "type the password for the username"
I need the App to share my blog posts on Facebook. I add my App to a plugin, and the plugin uses the Facebook API to share my posts on my Facebook pages and groups. To see how pages_manage_posts, pages_read_engagement, publish_to_groups, Groups API permissions are used in my App:
1. Navigate to "type your FS Poster App tab URL on your website"
2. Click the Add an App button and add the App: http://prntscr.com/tzdkw2
3. Then add a Facebook account to the plugin using a Facebook API. Head to Accounts > Facebook menu, and Click to the ADD AN ACCOUNT button: http://prntscr.com/txkboa
4. Select the App method, select the unapproved App (My FS Poster App), and click the ADD button: https://prnt.sc/u2xahm
5. Then, the plugin redirects you to the Facebook authorization page.
6. After authorizing your Facebook account, your pages and groups will be added to the plugin.
7. Now, if I publish a new blog post, the plugin will share it for me: http://prntscr.com/txoii5
The post shared on the group: http://prntscr.com/txo0ig
and the page: http://prntscr.com/txolea

And you need to create a screencast to provide it with Facebook. You can use any screen recorder app like Screencast.com to create the video. Here is what you need to deliver in the screencast.
Go to FS Poster > Apps > Facebook. Click the ADD AN APP button add your App credentials;
Go to FS Poster > Accounts > Facebook. Click the ADD AN ACCOUNT button;
Select your App from the select box and click the Add button.
Authorize your Facebook account.
Publish a post and show how FS Poster auto-posts the post;

And add the screencast as well as details to the complete form.

When your App is approved, you will be able to add many accounts and enjoy all the features of the Facebook App:
After adding your account, you need to add your App to the group settings to share posts on your managed groups. Go to your group > click More button > Edit Group settings > Add apps. Search your app name. Do not confuse other apps with your App. The sample app belongs to us.

You can set the plugin to share your post as link card view or upload your images. If your images are not visible on the link card view, you can debug your website on the Facebook developer page to see the reason.

You can also share your WordPress posts on Facebook groups on behalf of a Facebook page.

Add a Facebook account using the Personal App method.

Video tutorial: