Modifying the link resolver settings for Elsevier products including Science Direct, Engineering Village, EMBASE and Scopus are done through the Elsevier Admin tool.
As an example for Scopus, once logged in, navigate to the Link Resolver options and choose the option called OpenURL 1.0 SFX Standard. At the resulting page to edit this configuration, add your new LibKey Link URL as shown:
There are additional options on this page to name the link as you would like, add an image and more.
Note for Scopus: Selecting Yes for the option to "Use Default settings for this link" will result in not being able to change the custom image and alt text. So if you want to customize the image and alt text, then selecting No will allow you to make changes.
Other Notes: Scopus can take up to 24 hours for any updates to be reflected when you make a link resolver change and ScienceDirect can take up to 14 days.
Most other products from Elsevier including Clinical Key, Engineering Village and EMBASE require that a library reach out to the appropriate support desk to make a change. Here is a complete directory of all Elsevier support desks.