Answer
Use the External Search URL syntax instead of the plain action=get/key1= syntax: ifsapf:frmName?EXTERNAL_SEARCH=<KEY_FIELD>=<value> (for example ifsapf:frmCaseHeader?EXTERNAL_SEARCH=CASE_ID=176362). This syntax survives being pasted into third-party tools that silently reformat URL-encoded characters, continuing to work correctly where the plain action=get/key1= syntax breaks.