For anyone that may be stuck, we resolved this by adding the following below to the onload.js and applying the customization.
UpdatePlaceholders();
function UpdatePlaceholders() {
var attributesToUpdate = ["userNameInput", "passwordInput"];
var placeholderText = ["username", "Your Network Password"];
for (var i = 0; i < attributesToUpdate.length; i++) {
var node = document.getElementById(attributesToUpdate[i]);
if (node) {
var ua = navigator.userAgent;
if (ua != null &&
(ua.match(/MSIE 9.0/) != null ||
ua.match(/MSIE 8.0/) != null ||
ua.match(/MSIE 7.0/) != null)) {
var label = node.previousSibling;
if (label != null) {
label.value = placeholderText[i];
}
}
else {
node.placeholder = placeholderText[i];
}
}
}
}