In order to utilize APIs within Flow, they need to have been added in the API Configuration section. Once you've added an API endpoint in the integration section, it can be utilized across multiple flows.
The process for the API setup of the required endpoints is as follows:
Under Configuration in the panel on the left, select 'API Integration'.
Click the +New API button at the top.
Complete the required information in the panel on the right and Save.
After you've set up your API as described above, you can test whether your connection is correctly set up and that you receive the correct response as intended.
To test your API:
Select the API you want to test from the list in the API Integration section.
Select the Test tab in the panel on the right.
Complete the applicable information in the Query Params and Headers sections as shown below.
Click the 'Test' button and look in the Body tab whether your test was successful.