How to pass pseudo element to iFrame

Hello,

I want to be able to apply the following CSS to the textbox labels in the Moneris iFrame:

label.monerisInputLabel:after {
    content: "*";
    padding-left: 3px;
    color: #a91721;
}

But I do not see how to do this with the provided variables found at: 
https://developer.moneris.com/Documentation/NA/E-Commerce%20Solutions/Hosted%20Solutions/Hosted%20Tokenization

We want the labels for Card Number, Expiry and CVD fields to have a required field indicator (i.e. a red astrix) Please advise.