The question is a bit too general; and you treat the term "API" to widely. It depends on the service you want to use. It could be a Web Service; in this case, you would use the interface published by the owner of the server or you may have WSDL file, produce C# or VB.NET proxy code using WSDL.EXE and include obtained source code in your project.
In other cases, you may need to send your request in the URL and analyze the HTML or other content you receive in response to get the data you need, in the style of what is usually called "
Web scraping". Basically, you will need to use
HttpWebRequest/HttpWebResponse.
Please see my solution and discussions:
How to get the data from another site[
^],
get specific data from web page[
^].
—SA
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