If you do it this way, clicking on the button will open up outlook and create an email with as sendto-address the email adress of the user, setting him up to send an email to himself. Also you will depend on the user’s default mail-program.
Likely you are trying to create a button on a page that, upon being clicked, will send an email to email-address of the user, having the url of the page in the body of the email. If so, read on, if not, sorry, my bad.
To do that, first setup your application to be able to send out emails. For instance by downloading and installing https://appstore.home.mendix.com/link/app/259/ or https://appstore.home.mendix.com/link/app/2461/
Once having that set up, check out this forum question https://forum.mendix.com/link/questions/101036 on the options to create a share button.