What you are describing is the very last step in the process of implimenting a customized user routine that is not available as a standard option. If you are sure that you need a user routine to represent your material behavior, then you need to write the user programmable routine and link it into the standard code, before you can use it.
I would refer you to the MAPDL Programmer's Reference Manual available online for all these details.
Programmer's Reference (ansys.com)
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