Drag and Drop a new Static Structural analysis system onto the Project Schematic of Workbench.

Click on the Geometry Cell in Workbench.

Look at the Properties panel on the right side of the screen.

Analysis Type is 3D.  Change it to 2D.

Now open SpaceClaim and draw your surface on the XY plane.

When you open the Model cell in Workbench, once Mechanical opens, click on the Geometry item in the Outline.

In the Details window, you will be able to select Plane Strain, which has no thickness.

The above will work, but as a shortcut, if your geometry is already on the XY plane in your existing model, drag the exiting Geometry cell onto the new Geometry cell, to create a link, then try to change the Properties from 3D to 2D before you open the Model cell on the new model.