I will say that there are several ways but, i suggest trying this:
-Get into your own blueprint
-Right click and search for get “Get Default Window Mode”
-Then pull the return value (That green one) and write “Set”
-Find an element called “Set by -Ref Var”, this makes the window mode as a variable and sets it’s mode
,your blueprint must be like this:
Change it’s value as you like.
Tell me your results.