Don't quite understand what you mean by highlighted. Generally, when a field is selected, it is highlighted, so I'm not sure what you want here. Because the normal way to see something in Access highlighted, is to have it selected. How did you want the highlighting to show?
OK, that would be using Conditional Formatting. To get to Conditional Formatting, open the form is design view, right click on Customer Name, select Conditional Formatting (near the bottom of the pop up menu). Here, you can ask this thing to make the background yellow when ID = [Search Customer's Name] field.