Control

Control 1:
Expand Solution-Explorer > TwinCAT Project > SYSTEM > TcCOM Objects > Object (XtsProcessingUnit) > Object (XtsPart) > Object (AT2001-0250) > Data Pointer
Right-click on Control to open the context menu
Click on Change Link… in the context menu
Control 2:

The Attach Variable Control dialog box opens.

Expand Solution-Explorer > TwinCAT Project > I/O > Devices > Devices (EtherCAT) > Term 2 (AT2001-0250)
Select Control
Confirm with OK
Control 3:

A gray arrow on the variable indicates successful linking.

Further information can be found in chapter Linking details.