Access Google Analytics Client ID in Sticky Bar


How do I get the Google Analytics Client ID in a sticky bar? I can’t access cookies because the iframe is on a different domain:

I need the client ID to write it as hidden value in a form.

Is it possible to transfer information from the main page to the sticky bar? On the other hand, I connected the sticky bar to Google Analytics via Google Tag Manager. Is there perhaps a way to access it directly via the Google Tag Manager or GA? So via your own javascript code inside the sticky bar…

Thank you very much for your help!