Just to clarify, the database username and password entered during a Lansweeper installation are the credentials used to make connect to the SQL instance your Lansweeperdb is hosted on or create the database if it doesn't yet exist. These credentials are not then used to complete the connection string in the config file of the service you're installing. We intentionally don't do this as with a new installation an SA account is required, while for connecting your service to your database, a db_owner account (lansweeperuser) is used. If we did use the installation account you'd end up with an account set up for database connections that too high a level of permissions, which would be possible a security concern.
We are constantly evaluating our software though, which includes are installer. Being able to complete the connectionstring from the installer is something that we may be able to add in the future.