Is it possible to post a link to Reddit via URL?
For example for Facebook you can do
<a href="https://www.facebook.com/sharer/sharer.php?u=http://stackoverflow.com">
Share Stackoverflow on your profile!
</a>
Does Reddit have an equivalent endpoint I can hit to share a URL?
There are several ways to do this, depending on exactly what kind of application you have.
If you're making something interactive, you can take a user to the submit page with a URL and title already filled in. The following URL will open the reddit submit form with a link to this question:
If you just want people to be able to submit your site or blog post to reddit, you can use a reddit button on your page instead.
If you're writing an app or a script where you need to post a URL to reddit, you can use the /api/submit route in the reddit API. If the user that is submitting the link has less than 2 link karma, then a CAPTCHA will probably be given.