To comment on Reddit, you just need to open a post and select the text editor or "Add a comment" option.

Here’s how I use the Reddit mobile app to stay connected with my favorite communities. I walk through how to use the Reddit mobile app step-by-step — from logging in, browsing subreddits, upvoting ...

You can hyperlink on your Reddit posts or comments by using the icon that looks like a chain, which is available on both desktop and mobile devices.