Problem with client-side asp.net validator

I'm quite new to this and need some help with a custom asp.net custom validator.

I have a function that adds a validator dynamically to an input. For checkboxes I have written a custom validator.

function AddValidator(controlId, validationMessage, isCheckBox = false) {
    if (typeof (Page_Validators) == 'undefined') return;
    // first remove validator so we do't add more than 1
    removeValidator(controlId);
    // Create new validator
    var newValidator = document.createElement('span');
    newValidator.style.display = "none";
    newValidator.display = "Dynamic";
    newValidator.id = "RequiredFieldValidator" + controlId;
    newValidator.controltovalidate = controlId;
    newValidator.errormessage = "<a href='#" + controlId + "_label' onclick='javascript:scrollToAndFocus(\"" + controlId + "_label\"" + ",\"" + controlId + "\");return false'>" + validationMessage + "is required.</a>";
    newValidator.validationGroup = ""; // Set this if you have set ValidationGroup on the form
    newValidator.initialvalue = "";
    newValidator.evaluationfunction = (isCheckBox == true) ? CheckBoxCheckedValidator : RequiredFieldValidatorEvaluateIsValid;
    Page_Validators.push(newValidator);
    $("#" + controlId + '_label').parent().removeClass('info');
    $("#" + controlId + '_label').parent().addClass('info required');
}

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I then add the validator with this snippet

AddValidator($(this).attr("id"),"Validation Message ",true);

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And the custom function is here:

function CheckBoxCheckedValidator()
{
return $(this).prop("checked") ;
}

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What this is trying to do is to return false if the checkbox is not checked, and true if it is. But the control that is passed into this custom function is not the checkbox itself, rather it is the control created in the AddValidator function (i.e. RequiredFieldValidatorMyControlName )
So the question is, how do I get to the control itself, to check it's current checked status.
Thanks