First of all, sorry for reviving this thread but I wanted to share my experience just in case someone is google searching as I was a couple hours ago.
Thank you so much to all the people that helped in this forum, specially you, Rogerooo, because this was the option that actually worked for me but I had to make a modification in the order of the commands. Taking as an example Ashen:
--app %romfile%--mount --run "Ashen" --fullscreen
Hope this helps.
Thank you all again.