There are two different places where links can be inserted in the Slides editor:
1. Block links (recommended)The easiest way to add a link is to make the whole content block clickable. Click on a content block and look for the "Link" option in the left toolbar to get started.
2. Inline text links
To make inline text links point to an internal slide you can use the following format: #/<slidenumber> or #/<slidename>.
For example, #/3 would take you to the third slide and #/4/2 would take you to the second vertical slide underneath the fourth slide. Note that these numbers are zero-indexed meaning the first slide is numbered 0.