how to connect a Logitech mouse & keyboard before iMac 27 signs on

I have relocated and I have my iMac 27 with me but my wireless keyboard & mouse were packed in a separate box that was to remain with my iMac. But the movers accidentally packed my keyboard & mouse box with my household items and I won’t have access to these items for at least two to three weeks. I went to Best Buy and purchased a set of Logitech, Mac operable, wireless keyboard & mouse. I am trying to pair these accessories but when my iMac powers ON, I need to Sign-in before accessing Prefs to pain them. My question is this. How do I get to the Pref Settings without a paired keyboard & mouse?