There are three things you need to follow, first, you need to set up marketing channels, for this you can refer below link https://docs.adobe.com/content/help/en/analytics/components/marketing-channels/c-getting-started-mch...
If you are comfortable writing custom rules then you can write using this option Marketing Channel Processing Rules otherwise go with the auto-setup approach.
The second thing is, create an event for a registration form, and configure eVars at Launch.
Onclick of registration button submit you need to call registration event by passing the dataLayer JS object.
The third step is while building report in the workspace, you need to use your registration event as metric and First Touch Channel, Last Touch Channel as a dimension.
Refer this link for your analysis https://docs.adobe.com/content/help/en/analytics/components/marketing-channels/analyze-mc.html
