You didn't mention what uStore Version or Skin you are using. This script is tested in uStore 11.1
Many ways to do make a Product Mandatory using JavaScript.
This script should get you started add this script to the Store Front Javascript.
<script>
$( document ).ready(function() {
if(window.location.pathname.indexOf('/ustore/Cart') > -1){
var mandatoryProduct = 'MyMandatoryProduct';//Product Name
if ($('.productName').find('label:contains('+mandatoryProduct+')').parent().parent().parent().find('input[type=checkbox]:checked').length ==1 && $('.productName').find('label:contains('+mandatoryProduct+')').length== 1){
console.log('Product ' +mandatoryProduct+' in Cart')
//$('#ctl00_cphMainContent_BtnSubmit').attr("disabled", "disabled").css("pointer-events","none");
} else{
$('#ctl00_cphMainContent_BtnSubmit').attr("disabled", "disabled").css("pointer-events","none");
$('#trTop').after('<a href="/ustore/#storeid#/Category/#categoryid#/Product/#productid#/">Click here to add Mandatory Product</a>');//update url to your #storeid# #categoryid# #productid#
}
}
});
</script>