You can do that by using these steps:

1. Make sure you have the Website, Helpdesk, and Forum modules activated.

2. Go to you Helpdesk configuration, enable the Community Forum feature, and choose which forums you want to give this access to:

3. When a user creates a forum post, they click on the three dots and then on Create Ticket:

4. And then a ticket will be created in that Helpdesk team. Also, you can navigate to the forum post by clicking on the forum post from the chatter of the helpdesk ticket: