For best security, our recommandation is to use a non trused connection string. Create a new SQL Server user and only allow him access to the Bulksign database.
Using a Trusted Connection with Bulksign
If you want to use Bulksign with a trusted database connection string, the Bulksign IIS application pool and Bulksign Background service will need to be updated to run under the user which is allowed to access the database.
Please see this section on how this can be done