iRule HOST Header replace with dynamic content

I’ve written this on my phone so apologies if it’s not 100% correct as I’ve not tested it. This example will take the first field of the Host header up to th first “.” Which in your example is admin, but it could be anything. It then replaces the host header using the test2 host and adding the dynamic first field.

when HTTP_REQUEST {
    set hostField [getfield [HTTP::host] “.” 1] 
    HTTP::header replace "Host" ${hostField}.test2.mycompany.com
}