I would require the string for running IE 11 in Macbook Air M1 BigSur 11.2

If you are referring to Internet Explorer running on a Win11 virtual machine in Parallels, it cannot be done since Microsoft has obsoleted IE for several years now and has zero support for it in win11, especially for a Win11 ARM 64 virtual machine.

Microsoft Edge is now the browser for win11 machines.

IE, as in Internet Explorer?

Microsoft discontinued the Mac version so long ago that most Mac users have forgotten about it. Just about the same time that Apple came out with Safari.

Even on Windows, Microsoft did away with Internet Explorer in favor of a new Microsoft browser called Edge.

RE: "This is internal website not accessible over the internet."

I'd suggest contacting your IT Department and suggesting that they upgrade their internal Web site to work with modern browsers. At some point, they're going to want to move to newer (and hopefully more secure) clients – even if they're a Windows-only shop and plan to stay that way.

If it is truly only compatible with IE 11, chances are, you don't.

You may want to contact whoever runs the web site as IE is now dead. It is no longer supported by Microsoft on any platform.

In early versions of Safari, there was an easy way to make Safari return the user agent string for Internet Explorer 7.0, 8.0, or 9.0. I'm running the version of Safari that comes with High Sierra, and it does not offer selections for any version of Internet Explorer, although you can enter a custom string ("Other…").

https://osxdaily.com/2013/01/16/change-user-agent-chrome-safari-firefox/

Changing the string will not magically add features or bugs to Safari that may be present in the browser that you are telling it to pretend to be. So configuring Safari with the Internet Explorer 11 user string might NOT allow the original poster to successfully browse the internal IE11-only Web site using Safari.

You can't run Windows apps natively on an Apple Mac.

We are an Apple support community of users like you, not Apple support. If you wish to contact Apple support use the Support link at bottom left this page to get things started.

I know. But I have an old junker Windows laptop that isn't worth anything.

I know IE is long dead replaced by Edge, but I'm intrigued by what the OP says.

The Asus laptop has XP on it; it's that old.

Suddenly no response .....

There is no "IE 11 mode" in Safari.

A web site that requires only IE 11? What's the complete address? I'd like to try it.