Define the pre-strain by selecting the Geometry in the outline, select the SYS\body and insert an APDL command snippet using the INISTATE command (right-click -> Insert -> Commands). The following APDL code must be used:
cmsel,s,body,elem !Geometry selection (here body, to be adapted)
INISTATE,SET,DTYP,STRE !Set stress value, in Pa
INISTATE,DEFINE,,,,,500000,0,0,,, !Define the stress value (here, 0.5 MPa)