I think you might want to elaborate on your question, I can't really answer it without first asking follow up questions.
Do you want to have a promotion apply to one specific product?
What promotion are we talking about? Built in, custom? Entry level/order level/shipment?
Please provide more information if you want help! :)
Also don't forget to respond to your other threads if you found a solution or not, it's crucial for future visitors to just not come to unresolved threads where they might actually be resolved. It also helps us that try to come with suggestions to see if we were in the right track or not. Example in your previous thread
Hi to all,
I need to know Is there any way to add the promotion of a particular product?
Is there any way to add the existing promotion to the new product?
thank you In advance.