Global reference to external Javascript library in Dynamics 365

We want help on how to add a global reference to an external javascript library in Dynamics 365. The javascript library must load a visible html <div> tag on top of the Dynamic 365 page UI on every page load. That's all. We will later add some content into the <div> tag, but your job is just to show how loading of the <div> tag should be coded. Our ambition is to end up being able to distribute our solution to companies. We are reading about things like PowerApps and HTML Web Resources, but don't know what the right approach is a general distributable solution for Dynamics 365, so please advice if you have any knowledge on that.