public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.activity_main);
        super.onCreate(savedInstanceState);
        LoadPage();
    }
    public Button Btn;
    public void LoadPage(){
        Btn = (Button)findViewById(R.id.LoginBtn);
        Btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, WebPage.class);
                startActivity(intent);
                final EditText username = (EditText) findViewById(R.id.LoginTx);
                intent.putExtra("Username", username.getText().toString());
                final EditText password = (EditText) findViewById(R.id.PasswordTx);
                intent.putExtra("Password", password.getText().toString());
            }
        });
    }

public class WebPage extends AppCompatActivity {
static WebView mWebView;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    final String user = getIntent().getExtras().getString("Username");
    final String pwd = getIntent().getExtras().getString("Password");
    setContentView(R.layout.activity_web_page);
    this.requestWindowFeature(Window.FEATURE_NO_TITLE);
    mWebView = (WebView) findViewById(R.id.webView2);
    String url = "http://www.webnotes.cz/";
    mWebView.getSettings().setJavaScriptEnabled(true);
    mWebView.loadUrl(url);
    mWebView.setWebViewClient(new WebViewClient() {
        public void onPageFinished(WebView view, String url) {
            view.loadUrl("javascript:document.getElementById('UID0').value = '" +user+ "';document.getElementById('PASS0').value='" +pwd+ "';javascript:document.getElementById('butLogon').click();");
        }
    });
}