I don`t know where is the identity token on paypal sandbox. is the identity token same like api number ?
Visit the Profile via www.sandbox.paypal.com, then go to https://www.sandbox.paypal.com/businessmanage/preferences/website, turn on "auto return" and "payment data transfer" and then the identity token will appear under "Payment data transfer".
If it does not display PDT token then try to purge cookies related to *.paypal.com and try the same process again. I solved this issue by following this. I found this from comment for this answer.