D365 portal checkbox enable / disable

Checkbox is special input type into javascript. You can try the following for checkbox enable / disable

 //Disable  
 $("#std__assistenzarzt").attr('disabled', 'disabled');  
 $("#std__assistenzarzt").attr("readonly", "readonly");  
 //Enable  
 $("#std__assistenzarzt").removeAttr('disabled');  
 $("#std__assistenzarzt").removeAttr('readonly');