It is better to put the button on a page (example: your join us page). Then share the link of that page in the email. So the user goes to that page and then clicks on the button to pay.
However, we do an option to use PayPal hosted button with this plugin also. This will let you create a payment link that you can use in an email. The following documentation has info on how to create a PayPal hosted button for membership payment:
https://simple-membership-plugin.com/creating-a-paypal-button-for-your-membership-payment/