Right. You can only fill in the 3rd party designee using the Desktop program in Forms mode. You can't do it in the Online version.
You have to switch to Forms mode (click Forms in the upper right (left for Mac)) and select the Info Wks. Scroll down until you get to Part V Third Party designee: section. Check the YES box and then you can fill in the name, phone number and any 5 numbers for the PIN.
