If you will be live captioning your livestream, you can add a link to the website where your captions are displayed and this will appear underneath the media player during your stream. Clicking the link will open it in a new tab.
After creating your livestream channel, click on Stream settings and then on Edit. This will open up the edit livestream modal:
Add your URL for live captions in the field provided. Your captioning partner should be able to advise you on what this should be, but it will typically be a service like Stream Text.