how do I implement a MATCH() ... AGAINST syntax on my search box? I want to search the database using 3 to 4 words typed by the user on the search input.
You can't define full text indexes directly on OutSystems. So, if you want to do this you'll need to create the full text index directly on the database (and stage it yourself between environments).

In order to use the operators required for full text search, you'll need to write your own queries using the "SQL" widget instead of aggregates.