Yes. If you know, or after you have figured it out or found it.
Of course, it is often quite easy. Often you sign up first on the web site. It was also not hard to figure out, for example, for the Bose music app it is Bose.com.
However, I already mentioned the ExxonMobil app. You have to know it is not ExxonMobil.com but Exxon.com. Or take the app, which is called "Credit Card" that I use with a credit card that Fidelity Investments provides me through a company called Elan that is owned by US Bancorp. The website is neither Fidelity.com nor Elan.com nor USBancorp.com but myaccess.com. I have a couple of other apps like that.
Again, I can live with this now that I know how it works. A bit of hunting can turn up the appropriate URL. It just would have been a bit handier if IOS Password could save right from the app, especially since in many cases you can sign up right in the app without ever encountering the web site.