@agataLa The key that should be added should be your language service resource subscription key. I recently worked with another user with similar question, please see this thread for reference.
For the second question, I believe you are referring to using PVA with QnaMaker and a bot? In this case, the flow has a new connector for language service offering of custom question answering. This was also discussed in detail in another thread. However, I do not see a confirmation from the user if they were able to setup, it up successfully.
With the recent announcement which disables creation of new QnAMaker resource other services' documentation is yet to be updated with guidance on using the language service offering.
I hope this helps!!
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