Using platform exclusive code in a shared project library

I’m just going to make this the culmination of all my cross-platform dreads.

I need to use windows exclusive code in my shared project library DownUnder UI. Is a shared project library the format I should be using? I usually never make topics asking for help, but the format is new and it’s harder to find documentation (forum posts) explaining it. It is the only format that will allow me to use the content manager. I do like using the latest formats personally. For those of you that don’t know, it is platform-agnostic, it only contains the source files with barely any configuration, and it is treated as if it is a part of whatever project it’s added to. Aka it will inherit all things such as nuget packages.

Another question though, say I want to access the clipboard from my shared project library. I know there is a way to flag part of your code as only usable on one platform, I don’t know how to do that. Say I’m using Monogame’s cross platform template, without using any third party libraries, how would I access the windows forms clipboard?