iRule Redirect with URL Masking

Yes. You don't need to give port in client URL, users can access  https://mytestsite. I hope you own the domain for  mytestsite and VIP for it is configured on your F5. You would need to create a fqdn pool for the destination "214124124.sdsddasd.com" , port 10020 and map this to the mystestsite VIP. F5 will do the port translation. Use proxypass iRule mentioned in the above discussion to rewrite HOST header. Hope this helps.