Developer: Optingun
Price: Optingun offers free plan. Premium plans start from $9. More details can be found here
Integration Support: Get in contact with Optingun
Optingun helps online businesses to create amazing forms to convert website visitors into subscribers using action driven pop-ups and social opt-ins.
How it works
1. Create or log in to your Optingun account and click the Connect your apps button.
2. Select MailerLite from the list and enter your MailerLite API key.
3. Create a new form by choosing MailerLite as your autoresponder integration. Then also choose the group that you want your subscribers to be added to.
That's it, you're connected! Now all of your new subscribers will be automatically added to your selected group in your MailerLite account.