Hi Karishma,
You need to concatenate moodle site link ,user id ,password & what ever you need to add in body.
For site link you can use $CFG->wwwroot . For userid & passaword you can check $USer,$Page values. Just use print_object() ;exit; with these parameters to check the values. I am bit busy now a days & not getting time to debug this hope this will be helpful for you.
Example :
$body = "You have been enrolled to course"."
".$course->fullname."<br/><br/>"."Please login to start
your course."."<br/><br/>"."Thanks,"."<br/>Admin"."<br/>$CFG->wwwroot;