Web APIs work on two main principles
1. GET - you have a URL that may contain parameters. You use Web.Contents with the URL and get your data back from that server. Oftentimes the response is in JSON format that you then need to parse. You can run these steps separately or pipe them together. As you mentioned sometimes these servers require you to authenticate or to add othe header information.
...
URL = "http://xxx/tstat",
headers = [#"Content-Type"="application/json"],
web = Web.Contents(URL, Headers = headers, ManualStatusHandling = {404, 400}]),
result = Json.Document(web),
#"Converted to Table" = Record.ToTable(result)
...
2. PUT - this is when you have secret data like a password that you don't want to send in clear text, or commands and data that you want to send TO the API server. That requires you to prepare the payload, and put it in the request body. An example is here
...
URL = "http://xxx/tstat",
headers = [#"Content-Type"="application/json"],
data = Json.FromValue([tmode = 2,t_cool = Setpoint,hold = 0]),
web = Web.Contents(URL, [ Content = data, Headers = headers, ManualStatusHandling = {404, 400}]),
result = Json.Document(web),
#"Converted to Table" = Record.ToTable(result)
...
You see that there's no way to specify GET or PUT - instead the presence of a non-empty Content will automatically change the method from GET to PUT. The Json.FromValue part here has a lot of pixie dust - not only does it convert the parameters into usable format but it also binary encodes the payload.
This all happens in Power Query which works the same on the Power BI desktop and on the service (when you refresh your dataset it will run all the included Power Query scripts).
Your example is a GET request with a URL that has some dynamic parameters. You can concatenate the URL string yourself, or use the Query= function to swap your parameter values in as needed.
