Default checkbox "Subscripe to this topic" to new topic or reply to ON
You need to enable the "Topic subscription option on post editor"
More info here:
However, if you have any users from the EU, I don't recommend to enabled his option. as this is a fundamental breach of the GDPR regulation.
Please read this doc for "Right to restrict processing"
In case you want to say thank you !)
We'd really appreciate and be thankful if you leave a good review on plugin page. This is the best way to say thank you to this project and support team.