You need to create a piece of code that gets the button element and change it's class from btn
to btnDisabled
. I'm not sure if this is possible in a standard record page, though.
Most searches on this topic are about doing this functionality in a Visualforce page (like thisthis or that). And while this seems to be doable with JavaScript, in the Salesforce Community the suggestion is to do this with a custom Visualforce page.