$("button").on("click", function() {
    var Username = $("#Username").val();
    var Password = $("#Password").val();

    var data = { "Username": Username, "Password": Password };
    $.post("http://path.to/api/Action", data, function(data) {
        if (data.IsOk) {
            // REJOICE! The post succeeded
        }
        else {
            // cry because something went wrong
        }
    });
});

var Username = $("#Username").val();
var Password = $("#Password").val();

var data = { "Username": Username, "Password": Password };

$.ajax({
    url: "http://path.to/api/Action",
    data: data
})
.done(function( msg ) {
    alert("Data saved");
    getReceipts();
});