if you are using the advanced solver types (which are based on Code_Aster), you currently have only the option to either have a linear elastic material, or a multilinear elasto-plastic material. If you want to have a perfectly plastic material, e.g. bi-linear elasto-plastic, you have to create a CSV file for the stress-strain curve with 2 points, like this:
strain [m/m], stress[Pa]
and upload it for the von Mises Stress value:
The first point defines your yield point, the second one is there to make sure a perfectly plastic material is created and since for every table upload you need to have at least 2 points (to allow interpolation and extrapolation of the data).
The detailed settings like Interpolation and Extrapolation are not relevant in this case, since all lead to the same result, just be sure to not define the Right-side Extrapolation as excluded since this would stop the calculation whenever the strain would surpass the last point given in your stress-strain curve (you could then interpret this as the ultimate strain after which the material breaks).
I hope this is clear enough. If not don’t hesitate to ask further questions for clarification.