Hi Lander,
To achieve this you could create a deeplink to the approval microflow and add this as a hyperlink in the email. Then the manager can click on the link and follow the procedure in the browser https://docs.mendix.com/appstore/modules/deep-link
Alternatively, you can use a web push notification which can include a button that approves the request without having to open the browser https://marketplace.mendix.com/link/component/114331
Hope this helps