Help with an I-rule rewrite

I'm trying to use an I-rule to add the http hostname in the url to the beginning of the uri then rewrite the hostname. Example:

If the client request to a VIP is https://proxy-v2.api-np-c.newapps.com/<somepath>, I want the request to the server pool to be this https://proxy-v2.api-gw-np-a.newapps.com/proxy-v2.api-np-c.newapps.com/<somepath>

I've tried this and it didn't work:

when HTTP_REQUEST {
  if {[HTTP::uri] starts_with "/" } {
    HTTP::uri /[HTTP::host][HTTP::uri]
  }
switch -glob [HTTP::host] {
  "*.api-np-c.newapps.com*" {
         set http_host [HTTP::host]
         set startPos [string first .api-np-c $http_host]
         set endPos [expr $startPos + 11]
         set http_host [string replace $http_host $startPos $endPos .api-gw-np-a]
         HTTP::header replace Host "$http_host"
}
  }

}

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After some digging, I plan to try this next:

when HTTP_REQUEST {
  if {[HTTP::uri] starts_with "/" } {
    HTTP::uri /[HTTP::host][HTTP::uri]
  } else {
switch -glob [HTTP::host] {
  "*.api-np-c.newapps.com*" {
         set http_host [HTTP::host]
         set http_host [string map -nocase {".api-np-c" ".api-gw-np-a"} [HTTP::uri]]
         HTTP::header replace Host "$http_host"
}
}
}
}

As you can probably tell, I'm really new working with i-rules. Any help would be greatly appreciated!