Loadmill has several ways to compose full API scenarios - the most common is Loadmill Composer Chrome Extension. This extension allows you to compose your API test scenario while browsing your application. It can register any type of incoming and ongoing API calls from the server.
Make sure to create an account if you want to use it.
Once installed, you can locate the Loadmill Composer as part of the dev tools tab. Click Command+Option+J (Mac)/ Ctr+Shift+J (Windows) to open your dev tools and select the Loadmill Composer tab.
You can drag the Loadmill Composer tab for it to be more visible for you every time you open your dev tools