From wiki.visual-prolog.com

For downloading, there is a Windows API called URLDownloadToFile which can do the job.

In your program, add the lines:

class predicates
    uRLDownloadToFile : (handle, string URL, string File, unsigned Reserved, unsigned Callback) 
        -> unsigned Hresult language apicall.
class predicates
    doDownload : (string SourceURL, string Destfile) [used].
    doDownload(SourceURL, Destfile) :-
        Hresult = uRLDownloadToFile(nullHandle, SourceURL, Destfile, 0, 0),
        if winErrors::s_ok <> Hresult then
            exception::raise_nativeCallException("URLDownloadToFile", Hresult,
                [namedValue("SourceURL", string(SourceURL)), namedValue("Destfile", string(Destfile))])
        end if.