This is a feature of the BETA Canadian ruleset available if you turn on feature preview and use either the Canada 1 or Canada 2 rulesets. Once using the ruleset, the deferral exemption appears under the exemptions button on the HOS status page. Simply select it and enter the number of hours you are deferring. Annotations are added automatically, and the calculations will reflect the defferal.
Note: as this is a BETA feature we do not recommend it for production, and any errors or unexpected behavior should be addressed via your normal support mechanisms.