Office Add-In Support for Word 2013

Question

I've recently started developing an office add-in for Word. My add-In basically inserts html code into word and works as expected on Office 365.

Our customer still uses Office 2013 though, and unfortunately the add-in doesn't seem to work in his application. The Add-In can be displayed but the html-insert code doesn't work (nothing seems to happen onClick).
Some further research indicated that even the lowest version of the API is unsupported by Word 2013 (Docs: https://learn.microsoft.com/en-us/office/dev/add-ins/reference/requirement-sets/word-api-requirement-sets).

That being said, is there an option to make the add-in work for Word 2013 and if not, where can I find further ressources / documentation for add-in development for legacy versions (COM / VSTO)?