oncheck listener for checkbox in javascript

You can do this for the input:

<input type='checkbox' onchange='handleChange(this);'>Checkbox

And this for enabling the button:

function handleChange(checkbox) {
    if(checkbox.checked == true){
        document.getElementById("submit").removeAttribute("disabled");
    }else{
        document.getElementById("submit").setAttribute("disabled", "disabled");
   }
}

Try Below for your requirement using jQuery.

$('#checky').click(function(){
     
    if($(this).attr('checked') == false){
         $('#postme').attr("disabled","disabled");   
    }
    else {
        $('#postme').removeAttr('disabled');
    }
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.5.1/jquery.min.js"></script>
<input type="checkbox" checked="checked" id="checky"><a href="#">I agree to keep my Username and Password confidential and uphold 
  the integrity of this pharmacy</a>
<br>
<input type="submit" id="postme" value="submit">

Using JavaScript, Please refer This