Sorry, That is not available in the Online versions. You can only enter a 3rd Party Designee in the Desktop program using the Forms mode. 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 the 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.
I guess for Online you could print out the return and write it in and then mail the return.