The external wrench provided to the externalForce function is defined in the "ee" (rigid body on the tree for your case) body's coordinate system when you also input the joint configuration to the function. Optionally, you could entirely omit passing the the joint configuration input to the function, but then, you would have to define the external wrench spatially transformed in the robot's base body's coordinate system.
This is clarified in great detail in this example here https://www.mathworks.com/help/robotics/ug/compute-joint-torques-to-balance-an-endpoint-force-and-moment.html
I hope this example helps you in formulating your problem and in defining the external wrench of your use case.
If you have follow-up questions or require further clarifications please do not hesitate to reach out MathWorks Technical Support.
Best,
Karsh
