Login to your InfusionSoft account
Next to the InfusionSoft logo, there is a small down arrow, click that and it should open a drop down
Click on Application in left sidebar menu
Scroll down, under API section, you will find the Encrypted Key, copy that
Now go to your wp-admin > Opt-in Samurai > 3rd Party Integration
Click Activate under InfusionSoft logo
Enter the name of the APP on your InfusionSoft which you are using already
Enter the Encrypted Key in the API Key field and hit Save Changes button
More details on InfusionSoft API Key is on this help documentation