Hi @asmolyakov,
This is not a chat, so please be patient when you’re waiting for support. Support forums is a free support area where plugin developers help based on their free time, so it may take a few hours or 1-2 days to get some answers.
I’m sorry but the answer on your question is negative. There is no way to make the subscription mandatory and there is no option to make it checkbox.
But you can enable (make it checked) the subscription button by default, so people well automatically subscribe to their comments when they post a new comment. Just navigate to Dashboard > wpDiscuz > Settings > Subscription and User Following
tab and enable this option: “Keep checked the ‘Notify of new replies to this comment’ option by default”:

This option will keep enabled (checked) the subscription button in the comment form by default:
