In the below example a new field is added to the support ticket submission for via the
hd_form_message hook. After the submission the conversation is filtered and the field is added to the message body.

	<?php // don't include this line in your functions.php, since it already starts with it.
	/**
	* Adds the website field to the ticket submission form
	*
	*/
	function hsd_enter_website_field() {
	$conversation_view = ( isset( $_GET['conversation_id'] ) && '' !== $_GET['conversation_id'] );
	// only for new threads
	if ( $conversation_view ) {
	return;
	}
	?>
	<div class="form-group">
	<label for="hs_thread_website" class="hidden"><?php _e( 'Enter Website', 'help-scout-desk' ) ?></label>
	<input type="text" name="hs_thread_website" id="hs_thread_website" class="form-control" required="required" placeholder="<?php _e( 'http://yoursite.com', 'help-scout-desk' ) ?>"/>
	</div>
	<?php
	}
	add_action( 'hsd_form_message', 'hsd_enter_website_field' );
	/**
	* Filter what is sent to Help Scout and add the website to the thread message.
	* @param array $conversation The conversation array that is used to create the thread with Help Scout
	* @return array Conversation array.
	*/
	function maybe_add_website_to_message_footer( $conversation = array() ) {
	if ( ! isset( $_POST['hs_thread_website'] ) ) {
	return $conversation;
	}
	if ( ! isset( $conversation['threads'][0]['text'] ) ) {
	return $conversation;
	}
	$conversation['threads'][0]['text'] .= sprintf( __( '<p><b>Website:</b> %s</p>', 'help-scout-desk' ), $_POST['hs_thread_website'] ); // add website
	return $conversation;
	}
	add_action( 'hsd_create_conversation_fields', 'maybe_add_website_to_message_footer' );

view raw functions.php hosted with by GitHub

Adding your custom fields to the message body is the only viable solution since these “conversations” are essentially emails, and tags are used for the
conversation type. You can learn a bit more about this via the Help Scout v3.0 release post.