If a client has agreed to receive marketing communication from you then you can record this preference in the client profile
Toggling the "Marketing Opt-in" button will mark them as having opted in to receive marketing .
The first time you send a marketing message, Slick will automatically send a opt out message to your client with a link for them to click if they would like to opt out. This will be recorded in the client profile as above. This message will be charged at standard rates.
For more information about GDPR and your responsibilities as a Data Controller, please read out Blog Article.