How to attract forum users to active participation?

Question

Marketing

Good day, comrades.
There is a certain forum with one main direction.
Since some other directions are connected with this direction, there is very little information on which there is very little information on the Internet, visitors began to come to the forum with targeted queries from search engines.
But they don’t get beyond registration, or they only write in private messages, but they don’t want to in public topics.
Because of this, sections with secondary topics are empty =(
How to involve them in active participation?
So that they do not hesitate to create new topics and answer the rest.
PS There are no restrictions on creating new topics and answers.
Now I think for beginners to prohibit private messages and leave a signature that you can ask a question in the main forum, but I'm afraid they will completely leave the forum.
The audience is mostly male, 30+ years old, with an engineering specialty.