Set Notes section/tab invisible

Hello,

I've got a query which I cannot resolve myself.

Using CRM Online 2016 I have created a form with a Notes section/tab. This tab is not visible by default. I want to make this section/tab with Notes visible through a business rule (basically when a certain string of text is used in another field).

Unfortunately the Notes-field does not appear in my pull-down when creating the business rule.

What am I doing wrong?

Thanks in advance for the help!

Vinod