Can't figure out how to bypass password in F keys for rebooting

Hi! I bought a used Dell Latitude 5400 laptop computer from Ebay - it came last week, I think. But I wanted to install Windows 10 pro (I have an installation disk I got from Ebay some time back, and have used it successfully, before. I couldn't get it to install just by putting the disk in, but managed to get the installation going, by first making a separate partition on the computer. But we had to leave in a rush, during the installation, and left it home, assuming I could finish later. It was a mistake, as you can imagine. I tried out all the F keys, but now it's gotten so no page with options shows up. I see no options to choose, except for to fill in the password, which I never knew, nor do I know now, what it is. Is there any way to bypass that need to fill in a password in order for me to be able to select necessary commands? Now it just says, "Dell Latitude E5400 Series" - at first, and then a white screen shows up with this message: "This computer system, #21D2TK1-2A7B, is protected by a password authentication system." You cannot access the data on this computer without the correct password. Please type i the system or admiinistrator password and press Enter.

I will very much appreciate it if you can help me figure out how to solve this problem! Bye.