When you click “DHCP Client” under “IP”, you’ll get two tabs on the right side of the window - DHCP Client and DHCP Client Options.
On the second one, you add a new option (code=60, name=whatever you want to name it, value - whatever contents your ISP requires).
Then, on the first tab, you click the triangle pointing down
to the left of the clientid in the DHCP Options row, and choose the name you’ve added in the previous step in the added tab field.