In fact, it is just a group setting that restricts everyone from posting to the group apart from the administrator. But even being just a group, it allows you to run it similarly to the Telegram channel.
Unfortunately, you will have to create a separate discussion group and advertise it by yourself.
This setting can be found in the group controls under the Permissions option. In addition, it will only appear if all group members use the latest Signal version, otherwise, you won't be able to see it.