ADS-B Exchange currently offers two ways to connect to them for fetching tracking data:
As a subscription through RapidAPI, which requires you to register with RapidAPI, subscribe to ADSBEx's API and provide credit card details (details below), or
if you operate an ADS-B receive and are feeding ADS-B data to ADS-B Exchange, then you can request an API key directly from ADSBEx (details below)
Find them in Settings > Input Channels > ADS-B Exchange:
Enables usage of ADS-B Exchange as a live flight data channel. This setting is only available if you filled in an API key in the next field.
ADSBX Radar View
This link opens the ADSBX Radar View, in which you can check coverage.
Paste or enter your API Key into this edit field. See below for options how to come by an API key. A button "Test and Save key" appears when you enter the key: Click the button to have LiveTraffic send a test request to ADS-B Exchange for validation of the key. If the test succeeded the key is saved to your settings and the channel enabled, otherwise not.
The "eye" button toggles if the key is shown as clear text or hidden as stars.
...RAPID API requests left
If ADSBEx is running with a RapidAPI key, then LiveTraffic shows the remaining quota as reported by RapidAPI.
This way has been introduced in May 2019 and has originally been designed with plugins like LiveTraffic in mind, which need only a very specific access to tracking data, namely aircraft in a defined radius around a center location. Please see the available subscription plans. There might be a free option to test the plan for a limited number of requests. For other plans there is a monthly subscription fee. All fees go to ADS-B Exchange. LiveTraffic and its author are not affiliated with ADS-B Exchange and I don't receive any gratitudes etc.
LiveTraffic sends requests in regular intervals as configured in Live data refresh in the Advanced Settings. By default this is 20s or 3 requests per minute. If your quota is, say 360 requests per month, then this converts to 120 minutes = 2 hours of simulation time. Would you configure the refresh rate to be 10s, then LiveTraffic would send 6 requests per minute and the 360 request quota is emptied after just one hour. (See below regarding monitoring usage.)
To connect to ADS-B Exchange via RapidAPI do as follows:
Register with RapidAPI, click on the "Sign up" link on their homepage and follow instructions for creating an account. Registration by itself is free.
Go to the ADSBEx Flight Sim Traffic API pricing page and select a plan by clicking its "Subscribe" button. You will be asked for credit card details. (Tip: The last field is for your zip / post code.)
Once you are subscribed go to the "Endpoints" page of the ADSBEx Flight Sim Traffic API and find you API key in the field X-RapidAPI-Key consisting of a long series of 50 digits and letters. From the Code Snippets on the right hand side, the key can be easily copied, screenshot below.
Back in LiveTraffic, paste this key into Settings > Input Channels > ADS-B Exchange > API Key, see above.
Also, the number of remaining requests as per your subscription plan's quota is shown (red arrow in screenshot above).
Now you can re-enable the ADS-B Exchange channel and (re-)activate Show Live Aircraft, so that LiveTraffic starts requesting tracking data from ADS-B Exchange.
LiveTraffic shows the number of remaining API requests of your monthly quota in the Basic Settings, see the above screenshot, the red arrow points to the numbers. This information is returned by RapidAPI with every network responce received. It is not tracked by LiveTraffic. It really is RapidAPI's information. Hence it should be fairly accurate.
You can (and probably should) also monitor your usage directly at RapidAPI. RapidAPI has some documentation on monitoring API usage.
You can request an API key directly from ADS-B Exchange if and only if you are actively feeding ADS-B data into the ADS-B network by operating your own ADS-B receiver. ADS-B Exchange offers help on how to set up receivers.
If you feed ADS-B Exchange, the go to https://www.adsbexchange.com/myip/, make a screenshot of the output, which contains details of what you feed to ADSBEx, and request a key from ADSBEx via their contact page.
You will receive a key currently in the format ########-####-####-####-############. Paste this key into Settings > Input Channels > ADS-B Exchange > API Key, see above.