A php webhook endpoint that will append a multi-picklist with missing values. Also, returns the property update values in an easy way to use in a custom code action later in the workflow.

appending dropdown multi-picklist when value does not exist.php

<?php

// RETRIEVE THE CURRENT DROPDOWN VALUES FOR THE LINE-ITEM PROPERTY
$curl = curl_init();
curl_setopt_array($curl, array(
  CURLOPT_URL => 'https://api.hubapi.com/crm/v3/properties/deal/line_items',
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_ENCODING => '',
  CURLOPT_MAXREDIRS => 10,
  CURLOPT_TIMEOUT => 0,
  CURLOPT_FOLLOWLOCATION => true,
  CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
  CURLOPT_CUSTOMREQUEST => 'GET',
  CURLOPT_HTTPHEADER => array(
    'Authorization: Bearer XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
  ),
));

$response = curl_exec($curl);
curl_close($curl);

// PARSE THROUGH THE VALUES OF THE MULTI-DROPDOWN
$property_data = json_decode($response, true);
$options = $property_data['options'];
$count_options = count($options);
$values_array = array();
$y = 0;
while($y < $count_options) {
	$values_array[] = $property_data['options'][$y]['value'];
	$y++;
}

// GET POSTED JSON DATA
$json = file_get_contents('php://input');

// GET THE POSTED DATA FROM THE WEBHOOK
$data = json_decode($json, true);
$record_id = $data['hs_object_id'];
$line_item_array = $data['line_items_json'];
$count_line_items = count($line_item_array);

// CHECK IF THE LINE-ITEM DATA IS IN THE DROPDOWN LIST OR NOT, IF NOT ADD
$x = 0;
$missing_values_array = array();
while($x < $count_line_items) {
	$line_item = $data['line_items_json'][$x]['name'];	
	$line_item_str .= $data['line_items_json'][$x]['name'] . ';';    
	
	if (in_array($line_item, $values_array)) {
		// NEW ITEM IN DROPDOWN NOT NEEDED		
		// NO ACTION
	} else {
		// BUILD AN ARRAY TO ADD NEW LINE-ITEMS TO DROPDOWN
		$missing_values_array[] = $line_item;		
	}
	$x++;
}

$count_non_value = count($missing_values_array);

if($count_non_value > 0) {
	ob_start(); 	
	$count_value = count($values_array);
	$v = 0;
	while($v < $count_value) {								
		echo  '{';
			echo '"label": "'.$values_array[$v].'",';
			echo '"value": "'.$values_array[$v].'"';
		echo '},';				
		$v++;
	}
	$z = 0;
	while($z < $count_non_value) {
		echo  '{';
			echo '"label": "'.$missing_values_array[$z].'",';
			echo '"value": "'.$missing_values_array[$z].'"';
		echo '},';
		$z++;
	}

	// SET THE PAYLOAD AS A SINGLE VARIABLE
	$payload = ob_get_contents();
	ob_end_clean();
	ob_end_flush(); 
	
	$payload = rtrim($payload, ",");

	$curl = curl_init();

	curl_setopt_array($curl, array(
	CURLOPT_URL => 'https://api.hubapi.com/crm/v3/properties/deals/line_items',
	CURLOPT_RETURNTRANSFER => true,
	CURLOPT_ENCODING => '',
	CURLOPT_MAXREDIRS => 10,
	CURLOPT_TIMEOUT => 0,
	CURLOPT_FOLLOWLOCATION => true,
	CURLOPT_HTTP_VERSION => CURL_HTTP_VERSION_1_1,
	CURLOPT_CUSTOMREQUEST => 'PATCH',
	CURLOPT_POSTFIELDS =>'{
	"options": [
		'.$payload.'
	]
	}',
	CURLOPT_HTTPHEADER => array(
	'Authorization: Bearer XXXXXXXXXXXXXXXXXXXXXXXXXXXXXX',
	'Content-Type: application/json'
	),
	));

	$response = curl_exec($curl);
	curl_close($curl);	
}

echo '{';
echo '"line_items": "'. $line_item_str.'"';
echo '}';

?>