Is single opt-in available with MailerLite webform?

Yes, simply click on your webform and choose “OFF” next to Double opt-in:

double_optin

A small reminder: Double opt-in means that subscribers have to confirm their subscription before they’re added to your active list. Those who do not confirm their subscription stay under unconfirmed tab. Single opt-in means that subscriber goes directly to your group, “active” tab.