WEBSITE FORMS

You can do that with an iRule:

when HTTP_REQUEST {
    if { [HTTP::method] == POST } {
        HTTP::collect
    }
}
when HTTP_REQUEST_DATA {
    set name [ URI::query [HTTP::payload] "name" ]
    log local0. "Nname is $name"
}

Name in this case is the name of the form field so set that to be what you want.