found this article:
https://kindersitz-im-test.de/maxi-cosi-...

"Is there a base for the Maxi Cosi Citi?
No, there is only an Isofix base for the Maxi Cosi Cabriofix. The Maxi Cosi Citi and the Maxi Cosi Citi sps are attached with the vehicle's three-point belt in the opposite direction of travel. The attachment can be made on the back seat, but also with the airbag turned off on the passenger seat.