I have not tried this myself yet so I am not sure if it works, but I read that you can use the Execute Console Command node with the command: r.setres 1920x1080wf
You can change 1920x1080 to any resolution and I think it would make it windowed boarderless. Hope this helps.
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: