Use the STARTAGENTSERVICE property:
0 prevents the BESClient service from starting after installation
1 starts the BESClient service after installation (default behavior)
msiexec.exe /i BigFixAgent.msi /qn STARTAGENTSERVICE=0
setup.exe /s /v"/qn STARTAGENTSERVICE=0"
In our setup process, there is a guaranteed reboot after the BigFix client is installed, so the BigFix service starts at the next system startup. If you are doing this sequentially (e.g., in a batch file), can’t count on a reboot, and/or want to begin processing after your config changes, you could always start the service yourself, e.g.,
net start BESClient