First you can click on the service task marker
or navigate in the context menu and navigate to the "Service task" attribute.
The "Service task" attribute modal will appear
Now you can select an interface between all of those you created.
When you have selected an interface, select an operation between those available in the interface.
You can also edit the selected interface and the assigned operation by clicking on the button. Then if you wanna unassign the operation or the interface you can click on .