UDINT jsonCallback(UDINT userData, UDINT data) {
    IF userData = 0 AND data = 0 THEN
        RETURN ADR(jsonCallback);
    END_IF

    // Application code
}

pFunction := jsonCallback(0, 0);

unsigned long jsonCallback(myData_typ* userData, jsmn_callback_data* data) {
    // Application code
}

pFunction = &jsonCallback;