Since you don't have a display I am assuming you have the Apex 2016. If so:
After you perform the full reset on the Apex, go to it's local IP address and log in with the default un/pw.
If you haven't connected it to your WiFi yet: connect to the network the Apex is generating; type http://apex.local in your browser, it should then let you connect the Apex to your WiFi.
Once you have logged in to the local dashboard a box prompting you to generate a token should then be available
Yes, as @DarthSimon posted, holding the reset button for 20 seconds performs a full factory reset and will disassociate the Apex with the previous owner's account.