Been a while since I spoke to it in public, so here, my latest response to such a query:
Pretty much the only way to get the rights to do a book in a shared universe like Indy's or SW's is to be invited.
How it works is, the publisher leases the rights from the property owner -- and it's a spendy deal -- then hires a writer or writers to do the book(s).
It's a buyer's market and they have lots of folks from whom to choose. You don't call them, they call you.
An amateur's chances are slim and snowball. Doesn't matter how good you might be -- they don't know you.
Odds are overwhelming that if a publisher leases these rights, they are going to pick somebody with a clean and quick track record, i.e. somebody they already know and trust, to get the job done well and in a timely manner.
Unpublished writers or those with minimal credits usually aren't considered. There might be exceptions, but I haven't run into them. No need for the publisher to take the risk if they have somebody they already can reach out to can who can do the job. It's the devil you know versus the one you don't.
The best way to get noticed by the big franchises is to write material of your own that is similar -- space opera, fantasy, adventure, whatever, get that published, and thus have a calling card. You or your agent can then pitch to the universes you like, when they have some to hand out. That way, they can see what you can do, and if it's close enough to what they want.
If you have a novel done, you need to submit it to publishers. If they want it and want a cover quote, they'll find somebody who might be willing to do it. Agents help, though some houses will look at over-the-transom material. Get an up-to-date version or Writers Markets and look to see which ones are open.
I might also suggest that you go down the page and click on the PayPal button for my collection of essays, No Man But a Blockhead. There are some helpful hints on writing in it here and there. It's worth five bucks.
And good luck.