The BMW repair manual mentions the use of a special tool 2 412 612.
[Blockierte Grafik: https://www.newtis.info/tisv2/obj/1VncLrCqd7]
[Blockierte Grafik: https://www.newtis.info/tisv2/obj/1VncYH3846]
Set up exterior mirror (1) from the special tool 2 412 612.
Screw in countersunk screw (2) into the special tool 2 412 612 and brace the sleeve (3).