From the blueprint side, you can access FPaths::ProjectDir()
by using Rama’s Victory plugin:
https://forums.unrealengine.com/development-discussion/blueprint-visual-scripting/4014-39-rama-s-extra-blueprint-nodes-for-you-as-a-plugin-no-c-required
The Project directory can be access with the BP node “Victory BP Library / Paths / Victory Paths Game Root Directory”