Install the CBSubs Content integration, enable it, edit your CBSubs plans then on the Integrations > Extensions tab protect the entire PMS extension or just protect the URL to its compose feature, inbox, or whatever other pages you'd like to protect using By URL Part protection. Please review your CBSubs documentation for further details and examples.

I don't understand what you're asking. If you protect the URL to the extension then you should already have this. Click the button with SEF disabled. Now take the URL and trim it to match the below example format. Now add it to By URL Part protection and next time you click it should ask you to subscribe. Note you need to have the CBSubs Bot installed and enabled within Joomla plugin manager as well (included with your CBSubs package).

option=com_uddeim&task=new

Note the above is just an example of how to protect part of a URL.

It looks like you're using Super Messenger. You can protect the entire component by going to Integrations > Extensions > By Component then selecting "supermessenger" for "Components which access depends on this plan:".

Alternatively you can protect just the Compose URL within Super Messenger by adding the below URL to Integrations > Extensions > By URL Part.

option=com_supermessenger&task=invite&view=message

I'm not able to find those buttons on your site when logged in so not sure how they behave with all this, but should work fine as their destinations should be protected now (unless you're wanting to hide their display entirely?).