Wifi security code for Model No. R6400-100NAS

There's a support site with access to manuals, firmware, install guides, etc.
Go to https://www.netgear.com/support/
Type in your model number and from there it'll take you to the above.
Its a great resource to start with.

Look at the label on the R6400. This has the default wifi SSID and "Passphrase".

Maybe that is what you mean by "Wifi security code". That is not a label that appears in any Netgear manual.