Two things you need to do:
1) Create a Data Transformer
hich will let you use an input field instead of the default dropdown to the linked entity.
The Data Transformer basically transforms your input (in your case a username string) to actual user object that you can store with your entity. It will let you define an input field instead of the default dropdown to the linked entity. Have a look at the documentation, it's pretty straight forward.
2) Create the Ajax autocomplete
This can be done using any library you want (jQuery, Prototype etc.) on the client side, and server side you handle this in your Controller action and replies with either JSON or a rendered template. There are plenty of tutorials for this out there. The Ajax result could include a link to create a new user, if none is found.
The Data Transformer is what you need to focus on, and I would get that working before working on the UI.
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