Autocomplete form field from library

The user will need to select a name from a list of file names to complete a form. However, the list contains ~1000 items and it is not feasible to use the lookup field in the form. How do I create a field where the user can type and the field will autocomplete as the user is typing? I've found online that you need to use code in SP Designer. However, I am not sure where to add the code and what code to use. Any help would be appreciated.

Thanks.