Originally Posted by johnirvine

What you should do first is go to the Turkish Airlines office at TXL which, I believe, is situated around Gate 6. This is normally staffed by 2-3 very competent and helpful TK employees. Explain your problem to them and ask them if they could possibly link the two tickets - the SQ flight also operates as a TK codeshare. Behave humbly and apologise for not booking the onward leg on TK and they will most probably bend the rules. Once the tickets are linked, the agency staff at check-in will have no problem in sending your baggage to SIN. But, as noted above, you do need to go as early as possible to the SQ gate at IST to ensure that your baggage is loaded.