What kind of "Add-in" do you refer to? While the traditional approach of "loading on start-up" by using acad.lsp and/or loading per-document of acaddoc.lsp is still one of valid, viable options for add-ins of any type, if you are talking about loading DLLs developed with AutoCAD/Map .NET API, you can easily make the DLL add-in be loaded automatically on demands either by editing Windows registry (you can then build a setup with the *.reg file); or use autoloader, which has been available since Acad2013.
You can search the net for "AutoCAD app autoloading". There are a couple of them:
https://www.autodesk.com/autodesk-university/class/Depth-Look-AutoCAD-App-Autoloader-Module-2013
https://knowledge.autodesk.com/support/autocad/learn-explore/caas/sfdcarticles/sfdcarticles/How-to-a...
HTH