I have not done it so Im not 100% but I think
Make a new Layout that you want as the Slash Screen/Loading Screen
this can be just a static screen or your own custom loading animated bar or other widgets.
then in the project options
select this new layout as the "First Layout"
then select "nothing" in the Loader Style
then tick "Use Loader Layout"
This will use your first Layout as the Loader/Splash Screen(s)
You have to make sure the First layout is very very light and can load ultra quick otherwise you will have a black screen until it loads.
As I said though ive not done before so Im not sure how to trigger to go to next layout when all loaded or maybe it happens automatically