You can only assign keepers to traded picks only on the mobile app.
Once you enter those keepers in the mobile app, they will reflect properly on web, too.
If you attempt to enter them on web, you'll run into an error message.
Web Workaround
If you'd like to use our workaround below for web, you can, but we recommend that you use the mobile app instead.
Have the primary team owner de-select the desired player as a keeper. After that, the commissioner can manually select the draft tile on the board and choose "Set Player."
If you have any questions regarding this or run into any trouble, please let us know. You can email us directly at [email protected] anytime.