To link to the content of EndoNet use the EndoNet ID that is given on the detail pages in the format ENX0000, where X is a place holder for the type of the component (e. g. R for receptor or C for anatomical structure).
As URL for the linking append this ID to the detail page for this type of component.
For an hormone that would be:

http://endonet.bioinf.med.uni-goettingen.de/hormone/ENH00000

It is also possible to use the search of EndoNet to link to the right detail page. The URL should look like

http://endonet.bioinf.med.uni-goettingen.de/search/ENC00000