Login to your GetResponse Account. If you do not have any account already, signup for a FREE Trial here
Once logged in, hover over My Account on top right
Click Account Details under the My Account drop down
In Account Details page, click on API & OAuth
Look for heading “My GetResponse API key”
This shows the API key you need.
If you do not see your API key, click Regenerate under Actions button on right side.
Copy the API key
In your WordPress website, go to WP-Admin > WP Ultimate List > 3rd Party Integration
Click Activate button under GetResponse Logo
Paste your API Key in the form in bottom
Hit Save Changes button and you are done!