With the WordPress Newsletter plugin have the ability to place a subscription checkbox on your WordPress user registration form. When a new user registers at your WordPress website and tick this checkbox, the user will automatically be subscribed upon registration.
In order to enable (turn on) this new registration subscribe feature, go to the “Settings” section of the plugin and navigate to the “Subscription Settings” tab where you will notice the fields related to this feature as illustrated in the image above.
Set the “Registration Checkbox” setting to “Yes” to turn on this feature. Additionally, you may want to automatically check/tick the checkbox when someone goes to the registration form, therefore, set the “Auto check to subscribe” setting to “Yes” as well. The default value is “No”. Lastly, specify the mailing list to which these new WordPress users will be subscribed to. In case there are no mailing lists available in the select drop down menu, go to the “Lists” section of the plugin to create a new mailin glist.