Form Designer - How to start a Do loop after entering 3 characters in textField ? How to fired Up?

How to start a Do loop after entering 3 characters in the textField field? I know I can create a button to do this. But I want to type 3/4 characters and run a database search.

Can it be done? That would be a very useful feature.

I know how to do this for dropdown lists. But for textField it doesn’t work. I don’t know how I can read the textField properties (number of characters entered)

Can something similar be done for a dropdown list? My point is not to download data for the list beforehand. Only after entering 4 characters in the list, start the loop and download the data to the list.

Is any documentation available? Because I feel that I am moving on slippery ground and I have to guess and teast everything.

thank you in advance for your help.