You're taking name in document.getElementById() Your cb should be txt206451
(ID Attribute) not name attribute.
Or
You can have it by document.getElementsByName()
var cb = document.getElementsByName('field206451')[0]; // First one
OR
var cb = document.getElementById('txt206451');
And for setting values into hidden use document.getElementsByName() like following
var cb = document.getElementById('txt206451');
var label = document.getElementsByName('label206451')[0]; // Get the first one of index
console.log(label);
cb.addEventListener('change', function (evt) { // use change here. not neccessarily
if (this.checked) {
label.value = 'Thanks'
} else {
label.value = '0'
}
}, false);