How to do this for repeater within flexible content?

[amityweb]

I just cant get this to work with a flexible content field.

I have a flexible content field called "blocks"
Then in that I have a layout called "introduction"
Then in that I have a WYSIWYG field called "intro_copy"

I would like intro_copy to be searcheable, but entering the following in the "Custom fields to index:" box does nothing:

blocks_introduction_%_intro_copy
introduction_%_intro_copy

Do you have any idea please?
Thanks