Setting form fields to required not working in Adobe Acrobat XI Pro

Question

This should be a simple matter. However, I'm stuck in a rut with it, and cannot seem to get out.
I have created a form. All I need is for users to be prompted that they have to complete the form field when they try to tab or click out of that particular field where there needs to be information input. I have tried the following "on blur" java script code:
if (!event.value) {
    app.alert("This field is required. Please enter a value.");
    event.target.setFocus();
What continues to happen is when the user clicks out of that field, the error message comes up asking them to click "okay." The problem is that the error message does not go away after the user clicks it.
What I've learned is that if you right click on a field and click "set as required field," it doesn't work in the way I need it to. In fact, it hasn't been working at all for me.
Does anyone have a simple suggestion? I more than willing to try anything at this point.

Answer 1

OK, so the custom calculate script for field B could be something like:
(function () {
    // Get field values as strings
    var sA = getField("A").valueAsString;
    var sC = getField("C").valueAsString;
    var sD = getField("D").valueAsString;
    // Only perform the calculation if all fields are filled
    if (sA && sC && sD) {
        var sum = +sC + sD;
        event.value = sum;
        // Alert the user if the sum is more than A
        if (sum > +sA) {
            app.alert("Error message goes here");
    } else {
        // Blank this field if all fields are not filled in
        event.value = "";
Instead of (or in addition to) an app.alert popup, you could place this message in a read-only form field, and clear it if there is no error condition.