Hi Domenico,
Glad to help. If you have a subscription to the ANSYS Learning Hub (ALH), we have a course on User Programmable Features (UPFs) which should be helpful. There is also the MAPDL Programmer's Reference which has documentation on the procedures of compiling/linking and creating a subroutine. See Guide to User-Programmable Features within that reference manual. There is also an example of usermat implementation in Appendix C. User Material (UserMat) Subroutine Example.
If you let me know what OS and version of ANSYS you are using, I can point you to the documentation which lists the supported Intel Fortran and Microsoft Visual Studios versions.
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