In order for you to manage DNS with Hetzner DNS, you will have to save your Hetzner DNS API key to your Ploi account.
Head over to dns.hetzner.com and log in. After that, press the user icon at the top right, click on "API Tokens":
In here, enter a "Token name" and press "Create access token":
You will now be presented with a modal containing your API key:
Copy this token and go to your Ploi profile to "Integrations", scroll down until you see Hetzner DNS and enter the token there:
You will now be able to attach your sites to your Hetzner DNS API key and manage DNS records. To do this, head over to your site in Ploi and press the "Settings" tab, next scroll until you see "DNS Settings" and enter the right provider, and the domain name. Now a DNS tab will show up and you'll be able to manage DNS records directly from Ploi.