The encryption key is 128 bits and is held in registers QSPI.KEY[0] to QSPI.KEY[3]. The ordering of the bits in the four registers is documented in the nRF70 Series Product Specifications.
For example, the key 0xFFEEDDCCBBAA99887766554433221100 can be programmed using the Wi-Fi® Radio test command
wifi_radio_ficr_prog otp_write_params 0x110 0x33221100 0x77665544 0xBBAA9988
0xFFEEDDCC.