I’ve been wondering this since I learned about capsman in mikrotik (my mikrotik experience has only been 2 days).
Gateway/1st AP/capsman = hap-ac2
Repeater/cap = cap-ac
I setup the cap-ac 5ghz-wlan in station mode (as a client of the hap-ac2) as the backbone link. Gave the 5ghz-wlan interface 88.2 address.
Capsman setup as per (https://www.youtube.com/watch?v=OGFKfyzHnsM), with the CAP setup in the cap-ac as such.

Everything works great.
Next step is to trial:
- multiple ssids with multiple vlans in the cap-ac and figure out how clients will get address from the dhcp server setup for each vlan in the hap-ac2
- see if I can daisy-chain a second cap-ac from the first cap-ac (wireless mode set to st-bridge to repeat the backbone) to extend the capsman network wirelessly