Login to your ActiveCampaign account
On top left, click on your profile or username
In the drop down menu, click on My Settings
Click Developer from left side menu
The next page will show you API Access URL and API Key, copy both values
Now go to your WP-Admin > Opt-in Samurai > 3rd Party Integration
Click Activate under ActiveCampaign logo
Enter or paste both values respectively and hit Save Changes button