The version provided for Teensy users does not require a passcode (because there is no keypad) so it can be used for testing purposes without any buttons needed. To test it out you can just load the OnlyKey_standalone hex file and then start setting usernames, passwords, 2FA method with the Chrome App. To type out the credentials you can just press the pins on the Teensy with your finger. These are currently set to the following but you could change to other cap touch pins if you wanted.
#define BLINKPIN 13
#define TOUCHPIN1 01
#define TOUCHPIN2 15
#define TOUCHPIN3 16
#define TOUCHPIN4 17
#define TOUCHPIN5 22
#define TOUCHPIN6 23
Pressing the really small pins on the Teensy is obviously not good for much other than testing but you can try it out at least. As mentioned in the documentation it is not recommended to use this to actually log into your accounts, with the standalone test version there is no passcode required so this is not secure for anything but testing. In regards to wiring your own pads to the pins you could do that really any way you want to, solder a wire, a piece of copper tape, a paperclip etc. it works on capacitive touch so no pushbutton needed.
