How to use HelpDesk REST API

You can review API reference on this page.
HelpDesk REST API relies on SharePoint's REST API, so you can use OData queries with such parameters: $select, $expand, $filter, $top and $skiptoken.
First of all you must create an API key, this can be done on Settings -> API page.
To make an API query you must include an API key in the "X-HD-ApiKey" HTTP header.
All queries support SharePoint OData syntax, this page explains how to use OData syntax with SharePoint
By default all entities will contain only builtin fields, but you can query additional fields which will be placed in the "CustomFields" property.
By default all queries will return the first 50 items, but you can return to a maximum of 100 items with the $top query option. Also you can page through returned items with the $skiptoken query option. Example usage (note single quotes in $skiptoken): _api/v4/tickets?$top=100&$skiptoken='Paged=TRUE&p_ID=5'.
All Tickets list queries will not contain any attachments. To load particular ticket's attachments you must query this ticket and include query parameter "includeAttachments=true". All attachments will be returned as base64 encoded string.