In 2D mode, there's a "Layout" Button on the top. Click on it And select "Bottom right".
Then switch to another node and back to that button so the controls values update in the inspector (a little bug).
The layout button updates the values in "Anchor" and sets them to x=1,y=1. That means that this controls relative to the lower right of its parent container/viewport.
Go to "Rect -> Position" and substract 100 from the position values.
Done
You could also change the values in "Margin" (after setting the Anchor) but then you'd have to set both values accordingly.
When the Screen/Parent is resized then the button will be repositioned to always be 100,100 away from the bottom right.
For more info, read here:
https://docs.godotengine.org/en/3.1/getting_started/step_by_step/ui_introduction_to_the_ui_system.html#place-ui-elements-precisely-with-anchors