Modeless Javascript Prompt

How can I create a javascript prompt that floats in a fixed position on a page?

More specifically I am trying to create a floating prompt or dialog box th开发者_如何学运维at asks for a user ID and creates a cookie with that id number. It can't be accomplished with a simple javascript prompt because the user needs to be answer the prompt at will and still be able to access the parent window (ie modeless). Also, it must be accomplished by simply including a script (no html or divs) because it will be given to several clients and we want to keep it as simple for them as possible to implement.

This would require asynchronous javascript, correct? How would I go about creating this?