Do I need the knowledge of machine learning for this?

What is an auto complete tool?

when you start typing, it gives you guesses on what you are typing

so if I started typing “gr” it might give me “great” or “grass”

You don't need machine learning.

how would you suppose I do it then?

You need to find a way to get a list of all wanted words. Algorithm for searching the wanted items from the list should be easy to create.

ok