WoW classic has no profession API, but profession info can be fetched using GetTradeSkill... functions. This library provides methods that wraps the API calls and works the same way on both classic and retail. This makes it easy to create cross-platform addons.
If you are using BigWigMods packager add it under externals in .pkgmeta like this:
libs/LibStub:
url: https://repos.wowace.com/wow/libstub/trunk
tag: latest
libs/LibProfessions:
url: https://github.com/datagutten/wow-LibProfessions.git
tag: latest
libs/BM-utils:
url: https://github.com/Beast-Masters-addons/BM-utils.git
tag: v1.10
Otherwise, you need to add the content of this library in your addon folder as libs\LibProfessions
You also need to get the dependencies BM-Utils and LibStub and place them in the libs folder.
Add these lines to your addons toc file in this order:
libs\LibStub\LibStub.lua
libs\BM-utils\lib.xml
libs\LibProfessions\lib.xml
Include the library in your addon like this:
local professions = LibStub("LibProfessions-0")