1) You can obtain a sample of the material, perform a tensile test on it and create your own multilinear plasticity material model from the data. This might not be conservative because the material sample you obtained may have had a larger than typical yield strength, while the product might get built from material that is just above the minimum yield strength in the material specification.

2) You can build a conservative model that assumes the tangent modulus is zero and uses the minimum yield strength in the specification. If the design is made to support the maximum load with that material, then any product made to that design will outperform what the analysis predicted. That is a conservative result.