Populate dropdownlist through code (without postback)

 I have been searching the forum for this but I can not find exactly what I am looking for.  I currenly have a form set up where the user types in a reference code, the first three letters of the reference code says what kind of company the file belongs to e.g. PRV0999 means that the company is Private.  Based on the reference number that is enterted into the text box, I have dropdown box on the page that automatically populates with options that ONLY that company type can file.  So in other words if it starts with PUB, then only PUB options would show in the text box.
 The way I currently have this working is that when the user enters the PUB0999, the PUB is extracted through code and assigned to a text box which is the control for the dropdown list.
Example:
File: PUB000 ( When this gets updated, the PUB is extracted though code and assigned to company type)  Company Type: PUB (When this this u ...