Which I don't agree with. There's a school of thought that reaching out to another team about a player that you want to trade puts you in a weak position. But, that is only true if your options are limited.
I would be on the phone with Colorado saying: We have identified you as a team that might want to trade for Miller and has what we are looking for as a return in any such trade. You guys need young controllable pitching. Miller is that. We need 3B, OF, Middle Infielders, and Catchers, you guys have that.
We're not here to ask you for anything that you can't part with. For instance, we know you won't part with Arenado but since you have him, and under control for many more years, then we need McMahon as part of the deal. BUT, McMahon of course alone isn't enough. Since you have a pretty set OF, we would need Dahl. We also want Nunez who is a young catcher likely 3-4 years away to round out the deal.
And go from there. They already know you are talking with Chicago and you have a number of other conversations ongoing. It's how a deal is made. You don't ask for anything they CAN'T give up but you make it hurt enough and get quantity and quality.