The permission for posting is configured per category. So you can allow “replying” and “creating (topics)” only for trust_level_1 and above.
The “Security” tab is hidden by default; use the “advanced settings” toggle at the top to find it. Apart from that recent change in the interface, the documentation topic is very helpful: Understanding groups and category permissions
In all categories configured like the example above, visitors and users with TL0 can only read, but not post. To allow posting in the introduction topic, you need a category where TL0 users are allowed to post. I think it’s best not to allow other users to create new topics in there, since TL0 users would be allowed to post in those too.
So in this category, you allow replying for trust_level_0, but do not allow creating topics.
I’m just a little unsure if that’s really what you want to achieve. After all, users can reach TL1 just by reading, and then post in all categories. So there’s still no requirement to introduce yourself first.