Connecting your Moosend account with Optingun forms is very easy.
Go to Moosend dashboard and click on Account on left menu, and select API Key, there you will have your API credentials, copy the key.
Go to create or edit your forms and navigate to settings page, you will see a tab for integrations.
Click on Moosend icon from the App list there. If you have not connected your Moosend account yet then this will ask you for your Moosend API details, if API details area already saved then it will go to next step directly to list you Mailing lists.
Paste your API Keys and account subdomain in Optingun Moosend Integrations settings page, and click "Save and Connect Moosend", this will validate your API details and if they are correct it will connect to your account and load your Mailing lists.
Select a Mailing List from the select box and click "Add Integration", your new Integration will be listed below that settings box.
If you want to add leads from this Optingun form to a new Mailing list, go to Moosend dashboard and Create a new List. After creating new list in Moosend, comeback to Optingun dashboard form builder, and click "Refresh Lists" link, this will pull the new list that you have just created, select the list you want and click "Add Integration"
Save your form after all these changes. Once this form is saved, your leads from this form will go the Moosend Mailing List you selected. And thats all it is to connect Optingun forms with Moosend.
You can also watch this video tutorial about the same process.