Hello,
this is not possible at the moment. With -target file the DocTool always asks for the file name with the "Save as" dialog, there is no switch for the target file yet. It is on our to-do list.
What already works without a dialog is the target clipboard, which copies the captured image to the clipboard:
pdf24-DocTool.exe -mode screen -target clipboard "<screen>"
All modes and targets are described in the manual: https://creator.pdf24.org/manual/11/#11117-capture-the-screen