OKC

A PMK identifier is defined as

PMKID = HMAC-SHA1-128(PMK, "PMK Name" || AA || SPA)

PMK - PMK is derived at the end of successful 802.1X authentication or after PSK authentication
AA - Authenticator Address / BSSID of AP
SPA - Supplicant Address

-> AP1 forwards PMK1 and AA to AP2.

-> STA sends Reassociation request frame to AP2 with pmkid2 in the RSNIE.
-> AP2 calculates pnkid2 using pmkid1 + AA + SPA.
-> If pmkid2 is acceptable to AP2, it will send M1 frame and starts 4 way handshake.

Reference:
https://www.cwnp.com/uploads/802-11_rsn_ft.pdf