Hi @Edoardo_Polito, welcome back! 
What you are reading on that doc is about Retool Database on Retool Cloud. It's showing how to connect to your Retool Database outside of Retool.
To do this, just go to your Retool Database by clicking 'Database' on the nav bar:
Then, click where it says Retool Database and this dropdown will show up:
The Postgres Connection URL is the connection string the doc is referring to. To see it, click the eye button on the far right.
The link on the doc seems to be redirecting to the right page when we click 'connection string': Access Retool Database tables externally | Retool Docs. Where you should see the following:
