How to properly set up a Border

I’m developing the interface of my game completely in C++. For some reason there are not much information about UE4 UI in C++. So I come here with my question. My question is pretty straight forward. How do I set up an UBorder for an UImage?

My current code looks like this:

That doesn’t produce the result that I want:

235526-hotbar1.png

I tried setting a color to the brush:

But, that paint the hole box:

235528-hotbar2.png

I would greatly appreciate, if you guys could help me on this one and/or point me to an article or post that explain it.
Thanks :slight_smile: