novnc.user.js

// ==UserScript==
// @name           noVNC Paste
// @include        https://portal.arpnetworks.com/noVNC/console?*
// @version        1.0
// ==/UserScript==

function sendString () {
  var str = prompt("Please enter a string to paste");
  if (!str)
    return;

	f(str.split(""));
  function f(t) {
    const XK_Shift_L = 65505;
    var character = t.shift();
    var i=[];
    var code = character.charCodeAt();
    var needs_shift = character.match(/[A-Z!@#$%^&*()_+{}:\"<>?~|]/);
    if (needs_shift) {
      unsafeWindow.rfb.sendKey(XK_Shift_L,1);
    }
    unsafeWindow.rfb.sendKey(code,1);
    unsafeWindow.rfb.sendKey(code,0);
    if (needs_shift) {
      unsafeWindow.rfb.sendKey(XK_Shift_L,0);
    }

    if (t.length > 0) {
      setTimeout(function() {f(t);}, 10);
    }
  }	
}

unsafeWindow.sendString = exportFunction(sendString, unsafeWindow);

var btn = document.createElement( 'input' );
with( btn ) {
  setAttribute( 'onclick', 'sendString()' );
  setAttribute( 'value', 'Paste' );
  setAttribute( 'type', 'button' );
}

document.getElementById( 'noVNC_buttons' ).prepend( btn );