how can I program my WiFi 6 extender without going through the Nighthawk app

While it would be easier to answer if you'd specify which extender exactly, but all the Netgear extenders I've seen so far answer on http :// mywifiext . net
(easiest to use from a wired connection to its ethernet port, but also possible via WLAN (NETGEAR_EXT)). Details are explained in the corresponding (downloadable-) user manual, e.g. under the header "Connect with the NETGEAR installation assistant".