Download and Install Browser Extension here.
Click the 'extensions' icon in the toolbar > click three dots
beside Chromium Browser Automation > Click Options.
Copy below code and paste under 'Import' text field.
Select 'Root' then click on 'Import'.
Before running the extension, make sure you're already logged in to the facebook account you want to use for posting to groups.
While in this page, click on the chromium browser automation extension. (assign keyboard shortcut (chrome://extensions/shortcuts) or pin it if you haven't)
Expand 'group', click project (you will see your imported code).
Click 'PLAY'. Leave CryptoTab browser open (maximized in the background) while you're doing what you are doing.
Use new window (not new tab) if you are going to use cryptotab browser while extension is running. (Extension runs only in the first/main window).
To minimize the risk of facebook ban/restrictions:
Have as MANY Ad Copies and Links as you can.
You can shorten url(s) here. You can make a new link with the same destination URL by just adding a unique parameter at the end of your url. (e.g. {YOUR_URL_IF_WITH_?_SYMBOL}&i=1, {YOUR_URL_IF_WITHOUT_?_SYMBOL}?i=2. Note that 'i=1' and 'i=2' were the unique parameters used supposed having the same destination URL.)
Post to up to 100 fb groups only per day (posts without link) or up to 50 fb groups only per day (with link). (This is only based from my personal experience and I cannot guarantee the numbers since facebook can improve its algorithm anytime.)