HOW do I know what the server expects to create the request string that is passed to the HttpClient ?
The server is expecting the parameters that are supplied within the URL as it's a get request.
This is the url for example:
http://olibweb263.lau.edu.lb:7782/eolib263?sf_entry=programming&session=42091200&rs=&style=tiau&infile=presearch.glue&searcher=tiau.glue&sf_subentry=&sf_entry2=&sf_entry3=&nh=20&beforedate=&afterdate=&x=31&y=20
And so these are the parameters expected to have values: sf_entry, session, rs, style, infile, searcher etc.. when you say in the url: sf_entry=someValue this means you're assigning someValue as the value of the parameter: sf_entry where sf_entry's value is expected by the server.
You said by looking into the html form, according to Ayman:
By looking at the form. you check it's Action attribute, to which page it is directed at and then do your HTTP requests on that file from your http client. i.e. if the form is like this: <form method = "get" action = "post.php"> you know that you will be doing your http request against post.php file on the server.