Add Client Script: Mandatory Field Highlighter using proper ServiceNow methods

- Highlights empty mandatory fields with error messages
- Uses g_form.showFieldMsg() and hideFieldMsg() instead of DOM manipulation
- Includes onLoad script for initial highlighting
- Includes onChange script for real-time updates
- Follows ServiceNow best practices and official APIs

Author: Rajat32-op

Client-Side Components/Client Scripts/Mandatory Field Highlighter/README.md

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+# Mandatory Field Highlighter
+
+## Use Case / Requirement
+Highlight mandatory fields that are empty by showing error messages, making it easier for users to identify which required fields need to be completed.
+
+## Solution
+Two client scripts that work together:
+1. **onLoad script**: Shows error messages for empty mandatory fields when form loads
+2. **onChange script**: Updates error messages in real-time as users fill fields
+3. Uses proper ServiceNow methods instead of DOM manipulation
+
+## Implementation
+1. Create an **onLoad** client script with the code from `script.js`
+2. Create an **onChange** client script with the code from `onChange.js`
+3. Apply both scripts to the same table/form
+
+## Files
+- `script.js`: onLoad client script for initial highlighting
+- `onChange.js`: onChange client script for real-time updates
+
+## Notes
+- Uses `g_form.showFieldMsg()` and `g_form.hideFieldMsg()` methods
+- Follows ServiceNow best practices (no DOM manipulation)
+- Works with standard ServiceNow forms
+- Provides clear error messages for required fields

Client-Side Components/Client Scripts/Mandatory Field Highlighter/onChange.js

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+function onChange(control, oldValue, newValue, isLoading) {
+    if (isLoading) {
+        return;
+    }
+    
+    // Update highlighting for the changed field
+    var fieldName = control.fieldName;
+    
+    if (g_form.isMandatory(fieldName)) {
+        if (!newValue || newValue === '') {
+            // Show error message for empty mandatory field
+            g_form.showFieldMsg(fieldName, 'This field is required', 'error');
+        } else {
+            // Hide error message when field is filled
+            g_form.hideFieldMsg(fieldName);
+        }
+    }
+}

Client-Side Components/Client Scripts/Mandatory Field Highlighter/script.js

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+function onLoad() {
+    // Highlight mandatory fields that are empty using proper ServiceNow methods
+    highlightMandatoryFields();
+    
+    function highlightMandatoryFields() {
+        var allFields = g_form.getFieldNames();
+        
+        for (var i = 0; i < allFields.length; i++) {
+            var fieldName = allFields[i];
+            
+            // Check if field is mandatory and visible
+            if (g_form.isMandatory(fieldName) && g_form.isVisible(fieldName)) {
+                var fieldValue = g_form.getValue(fieldName);
+                
+                if (!fieldValue || fieldValue === '') {
+                    // Show warning message for empty mandatory fields
+                    g_form.showFieldMsg(fieldName, 'This field is required', 'error');
+                } else {
+                    // Clear any existing field messages
+                    g_form.hideFieldMsg(fieldName);
+                }
+            }
+        }
+    }
+    
+    // Store function globally so onChange scripts can call it
+    window.updateMandatoryHighlighting = highlightMandatoryFields;
+}