How can I use FastMM4 memory leak reporting in a Service?

As Lars Truijens notes, writing to a log file requires file system privileges. The default Local System account (i.e. when you do not set an account explicitly or logon in your code) has full access to the local file system but has no default network access.

FWIW - I typically develop my services as regular Windows apps until the main part of the logic is up and running and stable. Using a library like SvCom allows you to run your services as a regular desktop application or as a service without any code changes.

Yes, provided the account used to running the service has enough rights to write the log file.