Every database is instantiated with a database ID which is an unique identifier. There are a few ways to get this DB ID. (You may see this written as dbid.)
If you are using the Desktop client you will see the DB ID when you connect to the database and select Open and then Neo4j Browser.
If you are using the web browser-based client you'll see the DB ID on the databases page.
You can also check your logs and look for the connection URI which will be in the form of neo4j+s://<DB_ID>.databases.neo4j.io.