You can use URL encoding to encode the newline as %0A
.
mailto:[email protected]?subject=test&body=type%20your%0Amessage%20here
While the above appears to work in many cases, user olibre points out that the RFC governing the mailto URI scheme specifies that %0D%0A
(carriage return + line feed) should be used instead of %0A
(line feed). See also: Newline Representations.