There's two different places where links can be inserted:
1. Text links
If you want to add a link to text you can find the option for this inside of the text toolbar. Click on it and you will be prompted to specify a link.
2. Block links
It's also possible to add links to other types of blocks, like shapes and images. This option is available in the block configuration options which appear when a block is focused.
The following link format can be used to point to another slide in your presentation: #/<slidenumber>.
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.