Now, this sounds more like a workflow question. I'm not an expert in the "Sales Deal" object. However, you should be able to find a BOR that represents one. Once you find it, you can create a sub-object and then add your event and method there. Adding a method is very simple; you can select the function module name and the tool will generate the code for you.
If you can't find an object, you can create a custom object with the needed method. You should also explore creating an SE24 class instead of BOR. The only thing here is that your class needs to implement interface IF_WORKFLOW if you want to use it in with workflow objects.