Using a "share" button to send an email with a link in message

I have a coupon that I converted to pdf format. I put a button on the coupon to "share" the coupon via email with a link. I used the "mailto:" command for the link. When I click the button it works to open my email client but the receipient, subject and message body are blank.

I can enter the receipient and subject manually but I would like a way that the link to the coupon would automatically load into the messagae body.

Can someone help? Maybe a javascript code or hopefully something simple.

Mike