We can use the schedule  / calendar to create task with customers.

We can create and see the task pressing   on the quick access bar or from Commercial / Agenda

If we click on New Registration we will see the following window, where we will write the client's name as a related person, and fill in the details of the task, as the date, if we want the program to warn us etc.

Once filled in, we can click on "Insert" and the task will be created. When is done we can modify the task by accessing the schedule and clicking on the edit button or mark as completed by clicking on the green tick

or remove it by clicking on the red cross