Sending form data to an API URL (Postman app helped to test API)

page-subscribe.php

<?php 
    /*
    Template Name: Subscribe Page
    */
    get_header();
    
    if( isset($_POST['subscriber_email']) ){
		$api_url = 'http://API_URL';

		$subscriber_name = $_POST['subscriber_name'];

		if ( isset ($_POST['subscriber_surname']) ) {
			$subscriber_name .= ' ';
			$subscriber_name .= $_POST['subscriber_surname'];
		};

		$subscriber_zip = $_POST['subscriber_zip_code'];
		$subscriber_email = $_POST['subscriber_email'];

		$response = wp_remote_post( $api_url, array(
				    'method'      => 'POST',
				    'timeout'     => 45,
				    //'redirection' => 5,
				    //'httpversion' => '1.0',
				    'blocking'    => true,
				    'headers' => array('content-type' => 'application/json', 'authorization' => 'TOKEN_HERE'), //Token maybe needs some double slash escaping
				    'body'        	=> json_encode(array(
											        'email' => $subscriber_email,
											        'name' => $subscriber_name,
											        'zip_code' => $subscriber_zip
											    	)
				    							),
					    )
					);
					 

		$content_error = '';
    
    
    //Process server response
    
    //If request failed
		if ( is_wp_error( $response ) ) {
 
            $html = '<div id="post-error">';
                $html .= __( 'There was a problem retrieving the response from the server.', 'wprp-example' );
            $html .= '</div><!-- /#post-error -->';
         
        }
        else {
        
        //If request succedded process messages (There can be error messages from server although request alone is succeeded)
         
            $html = '<div id="post-messages">';
            $errorMessages=json_decode($response['body']);
            
            $message=$errorMessages->message;
            $status=$errorMessages->success;
            
            if ($status){
                $html .= '<p class="success">' . __( 'You have successfully subscribed to newsletter') . '</p>';
            }
            else{
                $html .= '<p>' . __( 'An error ocurred') . '</p>';
                foreach ($message as $messageText) {
                    $html .= '<p class="error">' . $messageText[0] . '</p>';
                }
            }
            $html .= '</div><!-- /#post-error -->';
        }
         
		//Replace some unwanted strings from server response 
		$content_error .= str_replace("newsletter subscriptions.", "", $html); ;

		}
?>

<?php //Show server response messages
      if( isset($content_error) ){ echo $content_error; }; ?>
			<form action="" method="POST">
			    <div class="form-elements">
			    	<div class="row">
			    		<div class="col-md-4">
			    			<label for="name" class="mandatory">Name</label>
							<input name="subscriber_name" value="<?php if( isset($_POST['subscriber_name']) ){ echo $_POST['subscriber_name']; }; ?>" type="text" id="name">
			    		</div>
			    		<div class="col-md-4">
			    			<label for="surname">Surname</label>
							<input name="subscriber_surname" value="<?php if( isset($_POST['subscriber_surname']) ){ echo $_POST['subscriber_surname']; }; ?>" type="text" id="surname">
			    		</div>
			    			
			    		<div class="col-md-4">
			    			<label for="zip_code" class="mandatory">Zip</label>
							<input name="subscriber_zip_code" value="<?php if( isset($_POST['subscriber_zip_code']) ){ echo $_POST['subscriber_zip_code']; }; ?>" type="text" id="zip_code">
			    		</div>
			    	</div>
			    	<div class="row">
			    		<div class="col-md-8">
			    			<label for="email" class="mandatory">Email</label>
							<input name="subscriber_email" value="<?php if( isset($_POST['subscriber_email']) ){ echo $_POST['subscriber_email']; }; ?>" type="text" id="email">
			    		</div>
			    		<div class="col-md-4">
			    			<button type="submit" class="cta-button">Submit</button>
			    		</div>
			    	</div>
			    </div>
			</form>