Downloading data from download tool via API is similar to ordering a Macdonald’s meal via Uber Eats.

Step 1: Choose your store : URL
The URL of an API tells download tool which website do you choose for downloading your data.
Take google geocoding as an example, its URL is https://maps.googleapis.com/maps/api/geocode/
Put this URL in Download tool first tab Basic: URL

Step 2:   pickup or delivery?
The output format of data is another parameter that needs to be determined and normally it is in the documentation. For google geocoding, the output format is XML or JSON. If JSON is chosen, the complete form of URL is then extended to https://maps.googleapis.com/maps/api/geocode/json

Step 3: Choose your meal
The meal in terms of google geocoding is the lat/lon of addresses you entered. This step is to add these address into the download tool.
According to documentation, if the address is 100 Harris Street, the whole URL is address=100+Harris+Street. In the download tool, you can either choose to add address=100+Harris+Street as a whole after https://maps.googleapis.com/maps/api/geocode/json which is
https://maps.googleapis.com/maps/api/geocode/json/address=100+Harris+Street or put this part in payload: choose address as name and choose the field contains 100+Harris+Street as the value

Step 4: Check out
The last step is entering your API-key. Same as above, you can either add your API key in URL by adding key=[Api-key] in URL after address= or put a key in Payload as name and your API-key as a value

Review:
URL: https://maps.googleapis.com/maps/api/geocode/json/address=100+Harris+Street/key=API-KEY OR
PAYLOAD   URL:https://maps.googleapis.com/maps/api/geocode/json
payload tab:                      name: address |value:100+Harris+Street
key         |      value:  API-KEY