Step 1: Go to 'Integrations' and click 'Marketing Emails' on the right.

Step 2: Click on connect button next to 'Mailerlite'. 

Step 3: Log in to your mailerlite account click on the right-hand dropdown menu to navigate to Integrations.

Step 4: Click the Use button next to Developer API.

Step 5: On the Developer API page, you will see the API key on top. Click on Copy next to your API key. 

Step 6: Now, go back to your sumopreneur account, and you will see a pop-up where you'll ask for Name and API key. 

Give a name to your integration. 

And paste the API Key you copy from mailerlite, click the updates button to connect the service.