You can add a link either by embedding the URL on the post or by adding the link to a text on the post.

To embed the URL, first copy the link and simply paste it on your post and press enter.

To add the link to a text, first select the text on your post and you will see a black rectangular pop up with B, i, a link icon, etc appearing right above your selected text. Click on the link icon and paste the link you copied on the field that appears. Press enter and that's it.

I hope you got it. If not let me know. I can probably make a video that would clarify it better.