This article has the answers you're looking for:
Configure MSSQL Auditor on a LEM Agent - SolarWinds Worldwide, LLC. Help and Support
Specifically:
- SolarWinds recommends using a domain-based account that has sysadmin privileges on the database, although the account only needs to have Execute permissions for any stored procedures that have the xp_trace prefix.
And
If you want to use an account other than the Local System Account to run MSSQL Auditor on your database server, select This Account from the Run Service As section, and provide the appropriate credentials.
This account must have the "runas a service" right, and have SA privileges.