But the manual only indicates the code of big airplanes, where can i get the code of the plane im looking for?
You ask for this:
the manual doesnt say how to put baggages cars like in cls md 80
Which, as your wrote it, means you wanted to know how to use baggage cars, instead of cargo loaders. Cargo loaders have ULD codes, if you put a double quote "" sign, it will use baggage cars, as you asked. Since you said "like in cls md80", the MD80 is another airplane that doesn't use ULD containers, so you don't need any code for the 737 too, because code = "" (double quotes) means "no ULD".
The manual indicates the codes for almost all popular airplanes that USE ULD containers, you explicitly asked for a 737, and the 737 DOES NOT use ULD containers, because its cargo door is too small, which is why there's no 737 listed, since in real world the 737 doesn't use them (like the 757, is not there not because the manual is not complete, but because the 757 doesn't use containers on the lower cargo doors).
So, again, since you asked how to put BAGGAGE cars (not ULD containers cars) on a 737, you need to set code = "", as explained on the manual AND on my previous post.