Could you use the percentage code for new line? %0A%0D
Edit:
Using %0A
gives a linebreak, and %0A%0D
pops in a blank line space.
An example:
https://dom.com/new-message?groupname=staff&title=Help&body=Please%20include%20all%20relevant%20details.%0A%0DOS:%0ADevice:%0ABrowser:%0A%0DThe%20problem: