If your goal is to default in a value into the input
whenever the select
changes and allow a user to update it freely, then a good choice is to use a manual subscription.
var ViewModel = function() {
this.selectedValue = ko.observable();
this.otherValue = ko.observable();
this.selectedValue.subscribe(function(newValue) {
//perform whatever logic that you need to determine how the other value should be populated based on the dropdown's current value (newValue)
this.otherValue(calculatedValue);
}, this);
};