So, let's say User is assigned task ID = abc123 and let's say your Task list is in a table called TaskTable, which have a column called TaskID among other columns.
You can send him a notification with a link like ms-apps:///providers/Microsoft.PowerApps/apps/<App Guid>?task=abc123
This "task" works as a variable name. You can add as many as you want, e.g., ms-apps:///providers/Microsoft.PowerApps/apps/<App Guid>?task=abc123&company=delta
will create two variables called task and company which you can get value with the function Param.
On the app, you can have a call like LookUp( TaskTable, TaskID = Param(task)), so you can filter from your list only the task with that specific id you've sent to user (if you're not familiar with filtering, take a look here here).
Naturally, that depends on how you app was architected, so you might have a better solution or different needs for that.
Please let me know if that doesn't answer your question.