Login to your HubSpot account
Click on your profile icon on top left
From the drop down, click Integrations
From the left sidebar, click “Get your HubSpot API Key”
Next page will show a button to View API Key, click it
Copy your API Key
Now go to your WordPress’ WP-Admin > Opt-in Samurai > 3rd Party Integration
Click Activate button under HubSpot logo
Enter / Paste the API Key in the form below
Hit Save Changes button