How to change https://lalala to https://lalala/somethingelse/file.aspx

There are a couple of ways to do it depending on what you want the users to see, if you don't mind them seeing the true IP I would suggest doing a simple 302 redirect using an iRule, similar to the below:

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