1. Click the "Settings" option located in the left side navigation menu.
2. The Account Settings menu will appear. Click the "Developer" option.
3. The Developer Settings page will load and will display your ActiveCampaign API URL and Key. You can copy and paste this information into your 3rd party application.
Here is an article in the ActiveCampaign Help section that walks you through this process. Scroll to the "How to obtain your ActiveCampaign API URL and Key" subheading.