To have the DEFAULT entry changed you must use the following syntax:
TSS ADD(STC) PROCNAME(DEFAULT) ACID(PROMPT)
DEFAULT is the reserved keyword to change the default entry within the STC record.
Using DEF you will add the following entry what is not what it is expected:
Default entry is not changed:
STC = *DEF* ACID = *BYPASS*
and new entry is added:
STC = DEF ACID = *PROMPT*