I found some issues in your mentioned JS code.
1) If you want to add any element after other HTML element then pick its parent and add as a child.
2) You created one function named "onload" and function can be called with "()". So, code should be "window.onload = onload();" instead of "window.onload = onload;"
function onload()
{
var btn = document.createElement('button');
btn.id= 'btnGo';
btn.type='button'
btn.onclick = 'setSelectedValue()';
btn.innerHTML = 'Go';
var body = document.getElementsByTagName("body")[0];
body.appendChild(btn);
}
window.onload = onload();