"anbei" is mostly used in letters or documents when talking about attachments:
Anbei sende ich Ihnen die Dokumente A, B und C.
Enclosed please find the documents A, B and C.
The phrase Luisa Neubauer used would probably more often put as
Nur so nebenbei.
And, as πάντα ῥεῖ correctly explained, it means something along the line of "by the way" or "just saying".
The sentiment of the whole tweet could probably been translated as
Dear @AndiScheuer, we've been picketing your department every Friday for only 46 weeks, but just in case you didn't notice yet: We're called "Fridays for Future", not "Fridays for Yesterday". Just sayin'. See you Friday!
"Fridays for Yesterday" is a mocking name that seems to be used by climate change sceptics in order to discredit the Fridays for Future movement.