Where is the WPS button on the access point?

The AirPort routers do not employ a WPS button and only offer a very minimum implementation of this security method.

You may get this to work by using the following steps within the AirPort Utility:

AirPort Utility > Select the AirPort Express > Manual Setup

• From the AirPort Utility menu bar (at the top of the OS X screen), select Base Station, then Add Wireless Clients
• Select "Allow client by: PIN," and then, select Continue.
• Enter the PIN provided by the printer, and then, click Continue.

not helpful if you have an older airport extreme

BUT

this is helpful!!!

actually connecting your airport extreme to an WPA access point printer works and is easy 😉

https://www.youtube.com/watch?v=YgjPBYThDeQ

Since you replied to a post I made in response to the OP's original post over 5 years ago, I'm not sure why you would find it not be "helpful." ... and what generation of the AirPort Extreme are you referring to as "older?" There were no less than 3 versions of the 802.11g models and 5 versions of the 802.11n Extreme base stations from 2003 up to 2011 ... so please clarify which you are referring to that won't support this option? Actually, if you read the OP's post it is not even possible to know if he is referring to an AirPort base station at all.

The link you provided does cover the other option "First Attempt," but I have yet seen conclusive data that it will work with all wireless network printers that employ WPA as a security means.

Also, what exactly is a "WPA access point printer?" Did you mean to say a network-ready printer that supports wireless connectivity using the WPA security method?

yes, the original post is old

the response only addresses airport express

no router is specified but the response addresses airport express

not all printers come with a PIN code

if you do a search on airport and WPS- it leads to this post

WPS (typo not WPA)

whether the 'first attempt' solution works in all cases is doubtful (but what solution works in all cases?)

i can confirm that the wireless set up for the canon pixma mx925 with the airport extreme (no directions are provided with the printer) works and took only a few minutes

anyone without expertise can implement it simply due to the very helpful video... apple support for dummies 😉