The Office Fabric UI docs don't seem to specify how to present inline/contextual help alongside TextFields:
E.g. is it smaller text below the field in some kind of container with a background color?
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I am in the process of designing an Outlook plugin and ran into the same issue. In Fabric, there seem to be no official guidelines for the help text unlike Material Design.
In the Fabric UI Design Toolkit, there are some guidelines for autosave and error states that appear beneath the text fields. In my opinion, we can follow a similar approach to the help text too. I found this example on a Softpedia News article that does it. Hope this helps!
Edit: attached a part of the linked image for convenience.