iRule Rewrite Question

Do you want to use an explicit redirect (as VFB mentions) or perform a transparent rewrite?

As usual, this is best controlled using a class match. I'll assume there are two conditions: 1. change host to marvel... and set the request-uri to /app/marvel (you say append, and if that's what you really mean, it's possible, but does require some changes); or 2. change host to kibana... and set the request-uri to /app/kibana. I further assume that the decision is made based on the request host. So, I'll create a data group (using an internal data-group here for illustration purposes) where each data group element key is the request hostname, and the value is either "marvel" or "kibana":

ltm data-group internal dg-host-based-redirect { 
    records { 
        "foo.bar.com" { data "marvel" } 
        "bin.baz.net" { data "kibana" } 
        "ankle.feet.com" { data "marvel" } 
    } 
    type string 
}

For explicit redirect (not tested!):

when RULE_INIT {
    set static::hbr_data_group "dg-host-based-redirect"
}

when HTTP_REQUEST {
    set m [class lookup [string tolower [HTTP::host]] $static::hbr_data_group]
    
    if { $m ne "" } {
        switch $m {
            "marvel" {
                HTTP::redirect "http://marvel.somedomain.com/app/marvel"
            }
            
            "kibana" {
                HTTP::redirect "http://kibana.somedomain.com/app/kibana"
            }
        }
    }
}

If you want to transparently change those (not tested!):

when RULE_INIT {
    set static::hbr_data_group "dg-host-based-redirect"
}

when HTTP_REQUEST {
    set m [class lookup [string tolower [HTTP::host]] $static::hbr_data_group]
    
    if { $m ne "" } {
        switch $m {
            "marvel" {
                HTTP::host "marvel.somedomain.com"
                HTTP::uri "/app/marvel"
            }
            
            "kibana" {
                HTTP::host "kibana.somedomain.com"
                HTTP::uri "/app/kibana"
            }
        }
    }
}