As far as I'm aware there is no way to do that.
AI gave me this. If you've got some way of doing it let me know!
Quote:
There’s no official way to “force” Starlink as the preferred network
Unlike switching mobile carriers or manually selecting networks (which you can do with cellular networks), Starlink satellite service isn’t exposed as a selectable carrier in the iPhone settings — it is only used when your phone cannot see Telstra’s normal mobile network. You can’t force it to prefer satellite if there’s Telstra coverage present.

