PAI is not triggered after I choose value in F4

Hello experts,

I fight with the following problem:

On my screen I use input field (ucf_s_coi_afd03_ct-xrproc_param) and I coded the F4 help for it manually

PROCESS ON VALUE-REQUEST.
  FIELD ucf_s_coi_afd03_ct-xrproc_param
      MODULE req_xrproc_param_2006.

After I press f4 on this field, the values are correctly listed but after I choose one value (double click), I would expect that the PAI is triggered but it is not. I have to press ENTER, only then.

Is there a way how to trigger the PAI after the value is chosen?

I found one way, but I don't want to use it:

If I change the input field into "List box" or "List box with key", then I can assign function code to it and if I do that, then it works - PAI is triggered after value is chosen. However, I don't want to use list box, I need standard input field.

Any ideas?

Regards,

Tomas