Below link would help you on how to create flow and call it from PowerApps passing paramaters.
Specific to notes creation, you can use below on button click or whatever:
'PowerApps-CreatenotesinD365'.Run(txtNoteTitle.Text, txtNoteDescription.Text, Regarding)
Here regarding would be guid of record to add note to.
Attached is snapshot of how flow will look like. Make sure to map yellow highlighted fields in flow.
I came here looking for the solution to my problem, adding attachments not just notes, but have resolved the issue and I hope my blog helps others.
I found an easier way to patch notes to work, use Regarding:GUID(<record-guid>) instead...
Read more here:
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Features releasing from October 2020 through March 2021