If 'Alexa, open my curtains' already works for you, then creating a routine containing the same command to be triggered daily at the needed time should solve your problem.
I can say "Alexa, set my alarm tomorrow at 7 in the morning". I wish I could say "Alexa, open my curtains at 6:45 in the morning tomorroe" instead of having to go to my cellphone and use the clunky Alexa app.
It just seems like a wasted opportunity for a routine. I'm sure a lot more things would be available if Amazon let developers use more features.
We're not asking to construct a complex schedule via voice command, the app is sufficient for that. Sometimes you just want your curtains opened at 6:45 tomorrow because something is happening and you just thought of it. That's would actually be convenient.