In Microsoft Planner, tasks are generally associated with a plan that is shared with a group, meaning that all members of that group can see the tasks within that plan. Currently, there is no feature that allows you to create a completely private task within a shared plan that only one person can view.
However, you can create private tasks in a personal plan. Each user has the option to create their own private tasks in their personal space, which will not be visible to others. This means if you want to send a task to someone privately, you would need to create a separate personal plan for that user or use their personal task space.
If you are using the "Send to Planner" feature from email, it will add tasks to the shared company plan, and those tasks will be visible to all members of that plan. Unfortunately, this does not support the privacy requirement you mentioned.