We will generate our own `Authorization` header, because this API uses a slightly obtuse method. The API uses what's known as "Basic Authentication", which is essentially `"Basic " + Base64["username:password"]`. Since the password doesn't appear to matter for this API, we can generate our header like so.

    auth = StringJoin["Basic ", 
      ExportString["MYKEY:", "Base64"]]

It's important to leave the `:` in place when you paste in your key.

And now we can read the results from the API:

    URLExecute[
     HTTPRequest[
      "https://api.companieshouse.gov.uk/company/00000006", <|
       "Headers" -> {"Authorization" -> auth}|>]]