iRule Redirect with URL Masking

Do you own https://mytestsite?

Then, yes, you can do it, but not via a "redirect", it's done via foward proxy. Essentially https://mytestsite is your vip and https://214124124.sdsddasd.com:10020 is your pool.

If you application requires hostheader to be 214124124.sdsddasd.com and not mytestsite to work, then you will need to also modify the host header on the wire. This function may not be needed, it depends on your app, it is a function popularly known as "ProxyPass". You can Google "ProxyPass irule" or alternatively do it with LTM Profiles, if such functionality is required.