Reference the connection by name, the database could be named differently in another environment.\ Note: you might want to add backticks to the database name if the name uses alternative characters.
use Illuminate\Database\Query\Expression;
$tableName = 'your_table';
$databaseName = DB::connection('another_database')->getDatabaseName();
$table->foreign('created_by', $tableName . '_created_by_foreign')
->references('id')->on(new Expression($databaseName . '.users'));