Could you post more information?

On a typewriter, I used to back up and underline whatever. I can't do that on my keyboard for my desktop computer. How can I underline ? Thanks for replying.

If this is when composing a message for email or a reply can only do that via a toolbar button if present.
This forum doesn't have a button in its toolbar to underline text, but you can use HTML code instead (<u>text</u> : text).
Other forums or email composing editors may accept other code if there isn't a toolbar button (usually an "U") or only accept plain text.

In addition to cor-el's reply, if you are making a webpage, you can use CSS or even HTML. I still don't understand what you meant in your original post. Having to underline a letter to get to a webpage? Could you explain if cor-el's is not what you are after?

the site starts with https:// and the s is underlined

That 's' shouldn't have to be underlined to make such a secure https link work, so you can ignore it.
It is probably emphasized to make it more noticeable that a secure connection is used.