@Taifun thanks for pointing out to this extension.
This should be the solution but only if I figure out how to use it 
I'm getting this error:
Argument #2 (com.google.appinventor.components.runtime.TextB...) to 'cn.zzq.KeyEvent.KeyEvent.Arrangement(com.google.appinventor.components.runtime.HVArrangement)' has wrong type (com.google.appinventor.components.runtime.TextBox) (expected: com.google.appinventor.components.runtime.HVArrangement)
What am I doing:
"Pocni" is the screen name
"txtRadniNalog" is the textbox where I want to catch the Tab key.
What am I doing wrong?
Could not find any documentation on how to use this extension.