Use the command M587 as shown below. Substitute the SSID and WiFi password for those of your network. Enter the exact characters of your network SSID and password, in the correct case. The simple double quote characters (not curly quotes) are needed. Once you press enter, wait for the 'ok' response.

M587 S"your-network-ssid" P"your-network-password"

Send M552 S1 to connect to WiFi. The Duet will confirm the connection and the IP address of the Duet controller(note the IP address for later). The blue LED on the WiFi module will flash as it connects, then should be on constantly once connected. If your router's SSID is hidden, or you want to connect to a specific SSID, send M552 S1 P"my_SSID"

If you get an error, check the network SSID and password you entered with the M587 command. Make sure that the WiFi is in idle mode before sending M587, with M552 S0. Send M587 to get a list of remembered networks, though this does not show the password.

At this point your Duet should be on the wifi network. The next step is to connect to the Duet Web Control (DWC)