This is an updated version of Sartak's Editor Autocomplete add-on, which only works with previous versions of Anki. If you aren't using Anki 2.1 or higher, refer to his original repo here. Additionally, this version solves the problem of autocompletes not showing up after the Anki 2.1.2x update and the exception that gets thrown when using symbols.
This add-on enhances the Add Card screen with autocomplete functionality. Just start typing and if an existing card has a value that matches what you've typed, it will be displayed inline in the editor for you. From there you can click the value to quickly set the field.
The autocomplete search will only search the exact note type and field that you're typing into (so you won't get results from other note types or even other fields in the same note). The search also only does prefix matching, like most other autocompletes.
Psst, you may also be interested in the similar Frozen Fields plugin. :)