login on non-wordpress page

I am concerned that by answering this question, I will have added to your confusion. Because I just don’t understand why you would want to do this.

More detail on what you want to do, and why, would be very helpful.

But, to answer your question, the HTML iframe tag might work. Though redirecting the “non-wordpress page” to the real WordPress Login page would seem to make more sense.

I already have an iframe set up but when I sign into it, the site open up in that iframe. It does not redirect to the full site.

Then I would suggest redirection.

Something like this:
<meta http-equiv="Refresh" content="0; URL=https://www.site.com/wp-login.php">