The write data FIFO is referring to the “wr_mem” bus in Stratix® 10 Serial Flash Mailbox Client Intel® FPGA IP Core. 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 Intel® Quartus® Prime 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 Stratix® 10 Serial Flash Mailbox Client Intel® FPGA IP Core User Guide.