Autofill WITHOUT mouse

Hello! I'm trying to use autofill without my mouse.

Right now I need to use the mouse and use the black cross to drag down to autofill with series.

However I want to do this with a shortcut on the keypad.

The closest I have come to is by:

Choose cell

Shift + Down (as many times as needed)

Ctrl + D

However when I do this, the first cell is only copied to the others. I want it to do it in a series.

For example:

First cell = "Test_1"

I want all the other cells other to auto-increase by one to "Test_2", "Test_3" etc.

Thanks!