How I can send a task as link via email to a user so that a user clicks on that

Run a task report in the process to get the task id numbers and use a rule to build a Url to the task. You can use that task report to get the assignees too.

One more approach is
1. Create a dummy boolean variable.
2. Make your task as Quick task and create an exception rule to skip the node if your dummy variable is true().
3. Use linktoquicktask() to get the URL of this Quicktask and save in PV
4. Set the true() value to the dummy variable
5. Just use PV used in step3 in your email node

Hope this helps!