Best way to add URI at end of URL

You can use the following irule to do that:

when HTTP_REQUEST {
   HTTP::redirect https://[HTTP::host]/mobile/cplogin.aspx
} 

Are you sure about that '@'? Anyway, assuming it should be a period '.';

when HTTP_REQUEST {
 if { [HTTP::uri]] equals "/" } {
 HTTP::redirect https://[[HTTP::host]]/mobile/cplogin.aspx }
}

Try this:

when HTTP_REQUEST {
    if { [HTTP::uri] equals "/" } {
        HTTP::redirect "https://[HTTP::host]/mobile/cplogin.aspx"
    }
}

Do you specifically mean "testsiteone@xyz.com", with an ampersand in the URL?

I meant https://testsiteone.xyz.com/mobile/cplogin.aspx

When would you use this if { [HTTP::uri] == "/"

This essentially means "if the URI is blank (you didn't enter a URI in the request), the redirect to the /mobile URI". If you don't specify a URI in the request, it will always at the very least equal "/". You need a conditional check like this to prevent a loop.