Hi is there a function to take the planned dates and store to the Validate Dates fields

Yes, there is : on planning screen, use button "store planned dates into requested and validated dates".
(on left side of print button)

(which I would consider equivalent to setting the baseline)

Exact !

I searched the manual without success of finding this type of feature

The user manual may not be up to date.
There is a complete refactor of user manual planned.

Is there a task setting that is required for this function to work? I tried it on a 2 task test project and it copied the dates for one task, but not the other?
Thanks

No, I can't remind any requirement tgo have it work.
Please give more hints to investigate.