OK. I create the rule, which seemed the most simple.
Now, I created this rules:
Incoming from "Partituren Dieveliedoe" (that's the account name)
Add to category: "PARTITUREN".
When I clicked create, I selected: apply the following rules to the emails in the Inbox.
It worked, all the emails sent to that account go the red square in the Categories column.
But it does NOT automatically add the sign to it when a new email arrives. Is there something I should do to make that work?