send POST method should use the iRules sideband method. The difficulty is whether your server POST does not require username, password or Token authentication about F5?

firstly, F5 whether can ping NGFW and telnet NGFW 80(assume NGFW api service port is 80)?

secondly, if F5 can not telnet NGFW 80,  F5 need to add network route to NGFW, make sure F5 can telnet NGFW 80

here is the code, NGFW(example NGFW ip is 10.0.0.10, api service port is 80) with no authentication for POST 

when ASM_REQUEST_DONE priority 500 {
    set asm_ip [ASM::client_ip]
    set asm_json "\{\"Client_address\":$asm_ip\}"
    set content_length [string length $asm_json]
    set data "POST /sample/post/json HTTP/1.0\r\nHost: test.com\r\nContent-type: application/json\r\nContent-Length: ${content_length}\r\n\r\n${asm_json}"
    if { [catch {connect -time 1000 -idle 30 -status conn_status 10.0.0.10:80} conn_id] == 0 && $conn_id ne "" } {
        log local0. "Connect returns: $conn_id and conn status: $conn_status"
        set send_bytes [send -timeout 1000 -status send_status $conn_id $data]
        log local0. "Sent $send_bytes with status $send_status"
        close $conn_id
        return
    } else {
        log local0. "Connection could not be established to NGFW"
    }
}

Because you haven't given the complete format of the call api of curl - v and the http response header, the following code uses iRulesLX to send two https posts, which is equivalent to the effect of twice curl https formats:

1. curl -sku 'admin:xt32112300' -X POST https://www.test.com/auth/login

NGFW Cookie is in HTTP response header "Set-Cookie"

so, the next https post is bring cookie in http request header "Cookie"

2. curl -skv https://www.test.com/sample/post/json -X POST -H "Content-Type: application/json" -H "Cookie: XXXXX" -d '{"Client_address": "x.x.x.x"}'

iRules code:

when ASM_REQUEST_DONE priority 500 {
    set asmip [ASM::client_ip]
    set handle [ILX::init "ilxlab3_pl" "ilxlab3_ext"]
    if { [catch {ILX::call $handle jsonPost $asmip} result] } {
        # Error handling
        log local0.error  "Client - [IP::client_addr], ILX failure: $result"  
        return
    } else {
        # Log NGFW second https response body
        log local0. "result is $result"
        return
    }
}

iRulesLX code:

'use strict' // Just for best practices
// Import modules here
var f5 = require('f5-nodejs');
//var qs = require('querystring'); // Used for parsing the POST data querystring
const https = require('https');


// Create an ILX server instance
var ilx = new f5.ILXServer();

// This method will transform POST data into JSON
ilx.addMethod('jsonPost', function (req, res) {
  var options = {
    //hostname: '10.20.20.52',
    hostname: 'www.test.com',
    port: 443,
    path: '/auth/login',
    method: 'POST',
    auth: 'admin:xt32112300',
    rejectUnauthorized: false
  };

  var postclientip = req.params()[0];
  let httpBody = '';
  const req_post = https.request(options, (res1) => {
    console.log('statusCode:', res1.statusCode);
    console.log('header', res1.headers['set-cookie'])
    console.log(Array.isArray(res1.headers['set-cookie']))
    
    if (Array.isArray(res1.headers['set-cookie'])) {
        var ngfw_cookie = res1.headers['set-cookie']
        var post_asm_ip = postclientip
        var post_data = JSON.stringify({
            'Client_address': post_asm_ip
        });
        options.headers = {
            'Content-Type': 'application/json',
            'Content-Length': post_data.length,
            'Cookie': ngfw_cookie
        };
        options.path = '/sample/post/json';
        console.log(JSON.stringify(options));
        
        var post_req = https.request(options, (res2) => {
            res2.on('data', (d) => {
                process.stdout.write(d);
                httpBody += d;
            });
            res2.on('end', ()=> {
                console.log(`response data: ${httpBody}`); 
                res.reply(['successful', httpBody]);
            });
        });
        post_req.on('error', (e) => {
            console.log(e);
            res.reply('failed bring cookie post')
        });
        post_req.write(post_data);
        post_req.end();
        
    }});

  req_post.on('error', (e) => {
        console.error(e);
  });
  req_post.end();
  // Turn postData object into JSON and return to TCL
  
});

//Start the ILX server
ilx.listen();

HI Aantat, what kind of system is the other system, and did you want to manage those messages from a remote system (like pull on system C from system A, push from system C to system B) or just send messages directly from ASM to other systems? More details on what you're trying to accomplish would be helpful, but either way, there's likely a solution we can work out together. Let me know!