$("#multilineinput").on('keyup',function(e) {
$("#detailsfield").val($(this).text()); //store content to input[type=hidden]
});
//optional - one line but wrap it
$("#multilineinput").on('keypress',function(e) {
if(e.which == 13) { //on enter
e.preventDefault(); //disallow newlines
// here comes your code to submit
}
});