I found different solutions to do it and I finally decided to merge them in a single "optimal" code.
During the typing it checks each new word (from the previous whitespace to the current pointer) and proposes to complete it with words included in a array. Predicted words are shown in a list under the input field and can be selected with mouse or with up/down/enter keys.
These are the sources considered to create this code:
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Much nicer Phoenix, thanksĀ
However to fit my personal needs I managed to use this slight modif of Lupo's code for working with several inputs in the same gui
; http://www.autoitscript.com/forum/topic/158070-autocomplete-input-text/
#include <GUIConstantsEx.au3>
#include <WinAPI.au3>
#include <ListBoxConstants.au3>
#include <WindowsConstants.au3>
#include <EditConstants.au3>
Global $Words1[20] = ["fight", "first", "fly", "third", "fire", "wall", "hi", "hello"