Add a size guide to products which belong to a specific category

Hello,

I'm trying to add a size guide only to products which belong to a specific category.
It can be in the form of a link or an image.

I tried using a read only custom field but it didn't work out.

Any suggestions?

Thank you in advance,
Yannis

Thank you,

yes, indeed I used the custom text to write some HTML!

Thank you,
Yannis