You can find your Button using Document.querySelectorAll().
It uses a standard CSS Attribute selector as input:
string inputButtonValue = "Send";
// Or string inputButtonValue = "Disconnect";
var func = $"document.querySelectorAll('input[value=\"{inputButtonValue}\"]')[0].click();";
var result = await webView21.CoreWebView2.ExecuteScriptAsync(func);
The compatible alternative is to loop the result of Document.getElementsByTagName(), which returns a collection of HTML elements:
string inputButtonValue = "Send";
var func = "var elms = document.getElementsByTagName('INPUT'); " +
"for (var i = 0; i < elms.length; i++) {" +
$"if (elms[i].value == '{inputButtonValue}') {{ " +
"elms[i].click(); break;" +
"};" +
"};";
var result = await webView21.CoreWebView2.ExecuteScriptAsync(func);
You can also use WebView2.ExecuteScriptAsync() instead of CoreWebView2.ExecuteScriptAsync(), but the former is tied to WinForms, the latter is not. In case portability could be an issue at some point.
You should await those methods, as shown here, (since both are async), but it's not stricly necessary in case you don't need to evaluate result.
return ussd_send()is not the INPUT element'sclassName, that'sbutton. The TAG name andvalueAttribute can be used to single it out.getElementsByNamewhich will return all elements with that name (here two). to return the first you can add[0]after it, like this:webView21.ExecuteScriptAsync("document.getElementsByName('send')[0].click();");.