Can a Link button send a user name and a password?

No. A Link button cannot sign you in to a website. The Link button puts Pipedrive field values and text into a URL and opens the URL. It cannot type data into the fields of a web page, for example a sign-in form. See Link button.

The best solution is a website that supports single sign-on. The website then signs you in automatically.

If the target page must know that a click came from OneClick, use a Signed link. The Signed link does not sign you in. It lets the target page verify the click. See Signed link.