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You want to create a form on the site that your resellers can fill to register themselves.
Please follow these steps to create the form:
You might want to send a notification (Email/SMS) when somebody fills the form. You can configure a new Notification or use an existing one in the Settings tab and set it up to be sent when the form is filled.
The final step is to provide the form on the header of the store to provide a quick link to your users. You can do that by adding a new navigation.
Click here to know more about forms.