https://gplifehacks.com/2018/07/03/custom-links-using-skype-for-business-from-within-dynamics-gp/
Shawn takes Custom Links a step farther and shows off launching Skype for Business from a custom link.
Custom Links – Using Skype for Business from within Dynamics GP
Archived post from 2018. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.