You would use Schedule 3. You would only put in your share of the amounts, so that you only report your share of the capital gains.

So, I am looking at filing in 2020 capital gains for 2019 on the co-ownership sale of a rental property.  When I test it in the 2018 software, on schedule 3, it only allows you to type in the Proceeds of deposition value, Adjusted Cost Base and Outlays.   We are 50/50 co-owners and have reported that since we had the rental. The final sale value was $275K, initial value was $250K  and outlays of $3500. So, if I do what you say, I would report Adjust Cost Base of $250K, Proceeds of deposition of $262,500 (half the gains) and outlays of $1750 (half) for each of us?

just to confirm... you will do this on the capital gains page? 

For example: I own 10% of the property, and my parents own the remaining 90%. The property sold for $100,000, and the adjusted cost base is $90,000. 

I would put $10,000 as my disposition and $9,000 as my adjusted cost base right? like what I have below? 

Yes, you got it. You only report your share.

I hope this was helpful