You will just need to reconstruct the link and parameter needed to reach the assignment page directly from the link. The challenge here is that, in the Email Tool, you will not be able to tell what is the exact activity instance id for you to point to.

Best is to still use User Notification plugin which will be triggered upon creation of assignment. It will be "aware" of its own assignment and has access to it. It only takes a few configurations to achieve what you want to.

User Notification

Correct Walter ! Its mean i can't do what i want in above, right? I just can implement the User Notification Plugin. Oh, so shame..How if i want to type different contents in the email? because what i know is with User Notification Plugin we CAN'T make different content for each activity. Thankyou and please answer me again Walter

You can make use of the WorkflowManager service class to retrieve the activity instance ID in your Email Tool plugin if you need to customize your email message specifically.