In the ELN there are two options to add a link to another experiment. Link an experiment on a section level or link an experiment on the experiment level. 

To link an experiment on experiment level click on the Link button

In the appeared window you can navigate to the experiment click Choose experiment to link the experiment. 

The experiment link is visible in both linked experiments 

The other option is to open a text-, procedure- or comment section and click on the Link experiment icon

Next, you can browse through all projects and studies to find the experiment you want to link. Click Choose Experiment to insert the link to the selected experiment in your text section.