The write data FIFO is referring to the “wr_mem” bus in the Serial Flash Mailbox Client Intel® FPGA IP core for Intel® Stratix® 10 FPGAs. To pre-store data into write data FIFO, you need to write data to the “wr_mem” bus. You may refer to the IP wr_mem’s base and end address in the Platform Designer for the start address and list of addresses that you can write into.
For more details about the write operation flow, you may refer to the Serial Flash Mailbox Client Intel® FPGA IP Core User Guide for Intel Stratix 10 FPGAs.