How to use Rules to populate reference product after creating a Commerce product?
Despite the limited information in your question, you should be able to create such rule (using the Rules module) like so:


Use a Rules Event that is related to "When Commerce product is created" (something similar to "After saving content"?).
Add a Rules Condition to make your referenced product field become available to use in a Rules Action. This condition can be either of those alternatives:


"Entity has field".
"Content is of type".

Add Rules Action "Set a data value" for your referenced product field.