@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    webView = (WebView) findViewById(R.id.webViewId);
    textView3 = (TextView) findViewById(R.id.textView3Id);
    button = (Button) findViewById(R.id.bt1Id);

   webView.loadUrl("https://site.com.br");


    WebSettings webSettings = webView.getSettings();
    webSettings.setJavaScriptEnabled(true);

    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {

            String executaScript1 = "document.getElementById('campoLogin').value = 'admin';" +
                                    "document.getElementById('campoSenha').value = 'admin';" +
                                    "document.getElementsByName('button')[0].click();";

            String executaScript2 = "document.getElementById('disponivel').value;";

            webView.loadUrl("JavaScript:" + executaScript1);