You may check that variable name from src/config.production.php or src/config.development.php file. For example, from demo2022 project which uses Linked table:
<?php
/**
* PHPMaker 2022 configuration file (Production)
*/
return [
"Databases" => [
"DB" => ["id" => "DB", "type" => "MYSQL", "qs" => "`", "qe" => "`", "host" => "localhost", "port" => "3306", "user" => "root", "password" => "mypass", "dbname" => "demo2022"],
"demo2" => ["id" => "demo2", "type" => "MYSQL", "qs" => "`", "qe" => "`", "host" => "localhost", "port" => "3306", "user" => "root", "password" => "Elvis56", "dbname" => "demo2"]
],
...
As you can see, DB is the database variable name for your main database, and demo2 is the database variable name for your linked database/table.