Nick here from Shopify.
Can I ask how you've been trying to add the WhatsApp feature so far? There are several WhatsApp chat apps from the Shopify App Store which you can add which might make it easier to install on your product page if you wanted to investigate that further. You can see some of the ones I like the best below:
I'm not familiar with coding personally, so this is the route I would look to if it was me. Do you think this could work for you?
No problem at all.
Generally, these apps will have an icon in the corner of all the pages of your store to enable someone visiting your store to message you. You can see an example of this from how one of the apps would show it below:
When you say a note in the product page, do you mean to display a note from WhatsApp or have the chat icon showing up? If you could elaborate a little more or show another screenshot what you mean, the one initially isn't too clear for me.
Can I ask what theme you are using? Since it looks to be right to left Hebrew on the page, could you be using a custom theme? If so, your best bet might be to contact the app developer directly about it as it could be complex and not something that's easily shared here. It would be out of my comfort zone also, so I am pointing you in the right direction of where I think you can best and most effectively get this completed.