I can’t think of a single term that captures everything you’ve described.
In the business world, there is a lot of jargon that, beyond being inapplicable outside the realm of business, might not even be understood in a different office or industry. That said, here is a term you might find useful:
If you were speaking about company A and specifically referring to the opportunity that a relationship with them presents for landing business with company B (of which they happen to be a subsidiary in your example), you could describe company A as an “in”.
chiefly US, informal
a way of becoming involved in something or of influencing someone
Source: Merriam-Webster’s Learner’s Dictionary, definition of “in”
This “in” is usually paired with “with”. Here are two equivalent examples:
We should really try to get Triscuit’s business. They could be the in we need with Kraft.
We should really try to get Triscuit’s business. They could be our in with Kraft.
You could also say that company A would allow you to get your foot in the door with company B.