To do this:-
First, open any form in the Ninja Forms editor…
… then click on the “Emails & Actions” tab to open the actions settings.
Next, click on the “+” button to add a new action…
… and select Noptin as the action.
Ninja forms will add the action and open the Noptin action settings. Use these settings to map the fields on your forms to Noptin fields.
For example, you can map the subscriber’s email to an email field on your form or the email address of the currently logged-in user.
The same applies to the subscriber’s name and any other custom field that you have set up.
Finally, save your changes.