The scratchpad for this device is not directly accessible. The part still has one, but it is used internally by the part.
In other 1-Wire devices, you are required to use the scratchpad to write to memory. The sequence would be to call WriteScratchpad followed by a ReadScratchpad (to make sure the write took place and to get the E/S byte back), and then finally calling CopyScratchpad which copies the scratchpad’s contents to memory.
With the DS2406, you do not have a scratchpad you can call directly, but you do have the Write Memory command with which you can only write 1 byte at a time (that is why the internal-only scratchpad is 8-bits).
The top of page 9 in the datasheet has some nice, concise verbiage on how to write to the part:
“The function flow for writing to the Data Memory and Status Memory is almost identical. After the appropriate write command has been issued, the bus master will send a two-byte starting address (TA1=(T7:T0), TA2=(T15:T8)) and a byte of data (D7:D0). A 16-bit CRC of the command byte, address bytes, and data byte is computed by the DS2406 and read back by the bus master to confirm that the correct command word, starting address, and data byte were received.”