Answer
Use the EXTERNAL_SEARCH mechanism in the target address instead of trying to pass the value as a normal parameter — the pattern is ifsapf:windowname?EXTERNAL_SEARCH=FIELD_NAME%3Dvalue, where %3D is the encoded equals sign, FIELD_NAME is the destination screen's field name, and value can be either a literal fixed value or a substitution reference like {&SOURCE_FIELD} (in braces, unencoded) pulling from the current screen. Multiple conditions can be chained together with %26 (encoded &) between them — this EXTERNAL_SEARCH syntax is the supported way to pass both fixed text and dynamic field values into a Custom Menu's target screen.