Conditional function

I'm using the Membership plugin on a site. On the page where users select their subscription we would like to add a couple paragraphs of text.

I've written this function, but it adds the text to every page. Could somebody help me figure out how to make this conditional so that it only modifies the page ID 72?

function add_before_content($content) {
return '<p>This is the message</p>'.$content;
}

Thanks!
David