The two options that jump to mind for me are:
1. Using Events
You could add two events. One where you set the output on just after the start of the test and then another where you turn it off 5 seconds later. This is likely the easiest route but there are only a few trigger conditions supported in the product at the moment - if you need others we'd love to know what would be helpful!.
2. Using Python
With the FlexLogger Python API, you could write a quick script that turns your output on whenever you want and then use the "sleep" function to wait 5 seconds before turning it off. This would give you full control over when to control the output during your test. You can find some example scripts to get started in the documentation (https://niflexlogger-automation.readthedocs.io/en/latest/getting_started.html#channels) and/or watch the "Automating FlexLogger with Python" video on FlexLogger's learning page (https://learn.ni.com/learning-paths/datalogging-with-flexlogger)
Eric - FlexLogger Product Manager