What material are you using?  Does that material have a S-N Curve?

For example, the Structural Steel that is automatically assigned to every model has an S-N Curve that is shown below.

The table shows the S-N curve for an Alternating Stress with 0 Mean Stress. Case (a) shows a cycle that goes from 0 to 650 MPa, so the Mean Stress is 325 MPa and the Alternating Stress is 325 MPa.

You could either do hand calculations using fatigue equations, or you could use the Ansys Fatigue Tool. What do you want to do?

If you want to use the Ansys Fatigue Tool, you need to apply a Force to the model that creates the nominal stress of S = 650 MPa.  Use the equation S = F/A where F is the force in Newtons, and A is the area in square meters and S is the stress in Pa. The area A is the width of the narrow center times the thickness of the material, where units are in meters.