Scripts

Could you please tell me how to insert a counter into the quest window?

As I understand, the counter represents the stages of completing a quest, right? Let's say, killing monsters. In that case, you'll need ProgressCommentsFileRef. ProgressCommentsFileRef is used if the quest consists of several stages (for example, killing 3 groups of monsters).
This screenshot shows where the quest stages are defined (they can be displayed as a counter)

The command that can be used to update the quest stages is SetObjectiveProgress(‘SCRIPT_NAME_OF_THE_QUEST’, progress);
progress is the sequential number in ProgressCommentsFileRef.

I took part of the answer from Jack_of_shadows' message and added some of my own. I hope I understood the question correctly.