HTML Form Submit Username, convert it, continue to system

[Wasim.sh]

I am having trouble coming up with a sollution to the following.I have an HTML page with a form that submits a Username. This then needs to be looked up against something and the username converted from what the customer has written to what is actually used in the logon system. The main logon is handled by a PL script that requires a submission from "USER" and "PASS" text fields.In other words Username -> Conversion Page -> "USER" -> Logged on to system.Any suggestions would be most helpful.