Hi I am looking for some help with an irule that can call out to an api

You can use this for call out to another http based service: https://clouddocs.f5.com/api/irules/HTTP-Super-SIDEBAND-Requestor-Client-Handles-Redirects-Cookies-Chunked-Transfer-APM-Access-etc.html

iRules have only string parsing capabilities, but if the json is simple enough, there is no need for a full json parser.

This thread reminds me, that I always wanted to write a simple json_get_key function for iRules...

As juergen said, I'd script a procedure that calls API via a sideband connection.
You can use this as a starting point: (it's a working code that I'm running on a customer) 

edit : if you want to debug it, i'd recommend logging $payload value to understand better what the api is returning 
from there you can use string operators syntax to match the intended KV pair and estract the value

when RULE_INIT {
  set static::calls_timeout 5
  set static::first_recv_timeout 50
  set static::first_recv_tries 25
}

proc apicall {ip port uri} {
  set api_res ""
  set retries 0	
  set api_req "GET $uri HTTP/1.1\r\nUser-Agent: F5 API service\r\nHost: TBD\r\n\r\n"

  set api_conn [connect -timeout $static::calls_timeout -status conn_status $ip:$port] 
  if {[send -timeout $static::calls_timeout $api_conn $api_req]} {
    while {$retries < $static::first_recv_tries} {
      set api_res [recv -timeout $static::first_recv_timeout -status recv_status $api_conn]

      if {$api_res ne ""} {
        if {[string match -nocase "*Content-Length: *" $api_res]} {
          set header_len [expr {[string first "\r\n\r\n" $api_res] + 4}]
          set payload_len [findstr [string tolower $api_res] "content-length: " 16 "\r"]
          set api_res_len [string length $api_res]
          if {$payload_len ne "" and $payload_len > 0 and [expr {$header_len + $payload_len}] != $api_res_len} {
            set api_res "$api_res[recv -peek -timeout $static::calls_timeout [expr {$header_len + $payload_len - $api_res_len}] $api_conn]"
            set api_res_len [string length $api_res]
          }
        }
        set payload [string range $api_res $header_len $api_res_len]
		
		#the two lines below are used to match a string in response. In this example i want to retrieve VALUE from key KEY, formatted as {"KEY":"VALUE"} in response 
        set search_start [string first "\{" $payload]
        set api_res_ret [string trim [string map {"\{\"KEY\":" ""} [string range $payload $search_start [expr {[string first "," $payload $search_start] - 1}]]] "\""]
        
		break
		
      } else {
        set retries [expr {$retries + 1}]
        log local0.debug "No response after $retries retries, T= [expr {$retries * $static::first_recv_timeout}]ms."}
        set api_res_ret "FAILED" 
      }
    }
  }

  close $api_conn
  return $api_res_ret
}

when HTTP_REQUEST {
   # < your code here >
   # set token [HTTP::header Authorization]
   # set uri "/api/scim/Me?attributes=$token"
   
   set key [call apicall $api_ip $api_port $uri]
   if {$key eq "FAILED"}{ 
     # api call failed, do something 
   } else {
   
   # < your code here >
HTTP::header replace Authorization ""
HTTP::header replace User-ID $euid
HTTP::header replace User-IDType "SYSTEMLOGIN"
 
   }
}

I started with Ca-valli solution  because i was more comfortable with it but  with all the extra irules ( sideband, helper sideband) I needed  to incorporate it seemed bulky and seemed difficult to document and  maintain by others. I decided to go with the irulelx  as the json parsing was easier and the whole setup once I understood how it works is easier to maintain. Unfortunately the app group has not tested it yet. I will update this post once they get around to testing it. 

Thanks for all the help on this guys!