Add the following code to your current theme’s function.php

 
// # Note : In this example {agent_email} is the merge tag you want to use when you configure the email or advanced email settings.
// # This should return the agent email. Supposing you have multiple agent email, you have to add them in 

function agent_additional_email_meta_tags( $tags ){
     $tags['{agent_email}'] = 'Assigned Agent Email';
     $tags['{agent_email_1}'] = 'Agent Email Ph#1';
     $tags['{agent_email_2}'] = 'Agent Email Ph#2';
     $tags['{agent_email_3}'] = 'Agent Email Ph#3';
     return $tags;
}
add_filter( 'wpc_email_meta_tags', 'agent_additional_email_meta_tags' );

// # Note: Add the following code in your current themes functions.php 
// # This code will enable you to fetch the email value for the merge tag you have used above 
// # The query should be the same as the meta key you have used in the custom fields.

function wpcargo_email_replace_shortcodes_list_agent( $replace_shortcodes, $post_id ){
	$delimiter = array("{", "}");
    $replace_shortcodes = array();
    if( !empty( wpcargo_email_shortcodes_list() ) ){
        foreach ( wpcargo_email_shortcodes_list() as $shortcode => $shortcode_label ) {
            $shortcode = trim( str_replace( $delimiter, '', $shortcode ) );
            if( $shortcode == wpcargo_track_meta() ){
                $replace_shortcodes[] = esc_html( get_the_title($post_id) );
            }elseif( $shortcode == 'admin_email' ){
                $replace_shortcodes[] = apply_filters( 'wpcargo_admin_notification_email_address', get_option('admin_email') );
            }elseif( $shortcode == 'site_name' ){
                $replace_shortcodes[] = esc_html( get_bloginfo('name') );
            }elseif( $shortcode == 'site_url' ){
                $replace_shortcodes[] = esc_url( get_bloginfo('url') );
            }elseif( $shortcode == 'status' ){
                $replace_shortcodes[] = esc_html( get_post_meta( $post_id, 'wpcargo_status', true ) );
            }elseif( $shortcode == 'wpcreg_client_email' ){
                $reg_shipper = (int) esc_html( get_post_meta( $post_id, 'registered_shipper', true ) );
                $user_info   = get_userdata($reg_shipper);
                $reg_email = '';
                if( $user_info ){
                    $reg_email = $user_info->user_email;
                }
                $replace_shortcodes[] = $reg_email;
            }elseif( $shortcode == 'agent_email' ){
                $agent = (int) esc_html( get_post_meta( $post_id, 'agent_fields', true ) );
                $user_info   = get_userdata($agent);
                $reg_email = '';
                if( $user_info ){
                    $reg_email = $user_info->user_email;
                }
                $replace_shortcodes[] = $reg_email;
            }elseif( $shortcode == 'agent_email_1' ){
                $reg_email= esc_html( get_post_meta( $post_id, 'agent-1', true ) );
                $replace_shortcodes[] = $reg_email;
            }elseif( $shortcode == 'agent_email_2' ){
                $reg_email= esc_html( get_post_meta( $post_id, 'agent-2', true ) );
                $replace_shortcodes[] = $reg_email;
            }elseif( $shortcode == 'agent_email_3' ){
                $reg_email= esc_html( get_post_meta( $post_id, 'agent-3', true ) );
                $replace_shortcodes[] = $reg_email;
            }else{
                $meta_value = maybe_unserialize( get_post_meta( $post_id, $shortcode, true ) );
                $meta_value = apply_filters( 'wpcargo_shortcode_meta_value', $meta_value, $shortcode, $post_id );
                if( is_array( $meta_value ) ){
                    $meta_value = implode(', ',$meta_value );
                }
                $replace_shortcodes[] = esc_html( $meta_value );
            }
        }
    }
	
	return $replace_shortcodes;
}

add_filter( 'wpcargo_email_replace_shortcodes_list', 'wpcargo_email_replace_shortcodes_list_agent', 10, 2 );