Disclaimer: I'm not affiliated with Teachable. I have a school at http://portablebiz.club and am active in the support group. But this is NOT an official teachable workaround. 

The checkout url support document from Teachable is here  https://support.teachable.com/hc/en-us/articles/219442688-Linking-Directly-to-the-Checkout-URL, but, it's currently missing the steps to create the URL for a free course.The same principles are true, however. It's a combination of your school id + product id (the price) + course id

Step 1: find the pieces to create your checkout URL

Product ID

The Product ID is found by navigating to your course > Pricing tab and looking for the ID number of the plan (in this case, the free plan)

In this case, the product ID is 19727

Course ID

You'll find the course ID up in the navigation when viewing any part of your course. 

In this case, the course ID is 15232

School ID

Your school id can be found in several places. You might already know it, from creating support tickets. But you can find it easily by clicking on the checkout page of your site. It's the first number after the sso.teachable URL (it might be 5 or 6 numbers long - mine is school id 918)

In this case, the school id is 918

Step 2: let's put it all together. (school + product + course)

https://sso.teachable.com/secure/918/checkout/confirmation?product_id=19727&course_id=15232

Click on the URL above, to test and see how it works. Then, go ahead and make your own

Copy the code from here, and replace the XXX with your own course ID's 

https://sso.teachable.com/secure/XXXXX/checkout/confirmation?product_id=XXXXX&course_id=XXXXX

Step 3: Make sure it works

Test it in an incognito browser, to make sure it's working the way you would expect it to (a private or incognito browser will give you a view of your school that's similar to what a new visitor would see, without needing to log out of your admin area)

That's it - you're done! Share it in your newsletters, or add it to checkout buttons on another website.

Advanced:

Want to use this same method to create URL's to courses with coupon codes and affiliate links? 

It's the exact same theory, just add in your affiliate number, and coupon code, like this:

http://portablebiz.club/courses/btc?product_id=38019&coupon_code=PBCWEB&affcode=youraffiliatecodehere    

You can read more at the teachable support document https://support.teachable.com/hc/en-us/articles/115000591407