A big part of PR is the relationship--heck that's half of it.. ;^)--so reaching out to the reporter with an email like, "Thanks so much about your great story on Company B. I'm working with them in a PR capacity (ONLY IF TRUE) and am happy to help with any other inquires. Here's the company website (URL) and press kit page (page with photos, contact info, basic stats, approved quotes, etc.) which you can use at any time. If you see anything missing that you'd like on there just let me know. For example, if there's a certain company boilerplate that the LA Times likes to add to these articles, [Company Name, Employee Count, Website, Founded Date,] I can be sure to get you that. Thanks again!"
You get the idea. You're not even asking for the link directly but instead building your relationship with them as a contact. The more you make it as a way to make their job easier, the better.
Again though, only do this in your capacity of PR relationship with the company. If it's beyond your scope and they have a PR person, forward along the article to them and say, "It'd be great if they linked to the company's website! (URL)" That person should already have the relationship in place.