Use an access token that has access to the offline event sets. To obtain an access token, follow the instructions listed here in the Facebook for Developers Offline Conversions API for creating an App. Select 'Direct Implementation' for your app type.
Follow the remainder of Facebook's instructions for creating a System User and token. Ensure you have selected 'ads_management' for the scope.
Hightouch allows you to pass data to the default properties and the custom event properties of a Facebook offline event. Hightouch expects the event_time to be a standard ISO 8601 format. Hightouch will automatically convert this to the unix time that Facebook expects.