No, 765 form does NOT ask for OPT EAD approval date.
instead, what it really asks for is: "the date you applied for employment
authorization". if you put your mouse pointer on the field for several
seconds, some instruction info will pop out, showing above info.
So I put the date USCIS received my OPT application (on the receipt notice).
if you don't have it now, I think just put your mail date. This is
information IO can very easily find out in their system; so I think just
giving them an approximate date will be fine.
bless us all!