Basic applications without external dependencies may be able to build without any Preview configuration files, however more-likely-than-not you will need to define some configuration
Preview will always look for its configuration file in .preview/config.yml.
The most likely configuration you will have to provide are service dependencies. Preview is capable of deploying any Helm chart as a dependency, so long as the chart does not violate any current Preview restrictions.
As a basic example, an application which requires a PostgreSQL dependency, accessible with the hostname postgresql, and using a values file at .preview/chart-values/postgresql.yaml would be specified as: