A value of 1 will auto-select the destination, so you need to rename your platform to a number greater than 1. For example, "2 - Enterprise" will work, and will send rockets to it with a signal value of 2. Make sure the destination in the gui is set to "Auto" or it'll override the signal-specified destination with that one.
Admittedly, this mechanism is a bit wonky and is mostly a relic from before this mod had multi-signal mode, so you might also want to look into that.