In that case, you can create a mail script named: linkforreport as below.
(function runMailScript(/* GlideRecord */ current, /* TemplatePrinter */ template,
/* Optional EmailOutbound */ email, /* Optional GlideRecord */ email_action,
/* Optional GlideRecord */ event) {
var instance_url = gs.getProperty('glide.servlet.uri');
template.print("<a title='Click here for report' href='"+ instance_url + "remaining URL here in quotes"' target=_blank>Click here for report</a>");
})(current, template, email, email_action, event);
Call the mail scrip in the message in format
${mail_script:linkforreport}
