You need to send data to the web page using the POST method, as described in this stackoverflow article.

THe equivalent GET is something like:

http://www.csbio.sjtu.edu.cn/cgi-bin/Signal3L.cgi?R2=Plant&S1=MKMASSLA
+FLLLNFHVSLFLVQLLTPCSAQFSVLGPSGPILAMVGEDADLPCHLFPTM SAETMELRWVSSSLRQVV
+NVYADGKEVEDRQSAPYRGRTSIL&B1=Submit
[download]

which did not work on your site because it requires POST.

After the POST is sent, you need to write code to parse the response.