Set Input Value on Select Change

Part of the form() method:

->schema([
    Forms\Components\Select::make('product_id')
        ->relationship('product', 'name')
        ->live()
        ->afterStateUpdated(function (Get $get, Set $set) {
            $set('price', Product::find($get('product_id'))->price);
        })
        ->required(),
    Forms\Components\TextInput::make('price')
        ->required()
        ->numeric()
        ->prefix('$'),

In the afterStateUpdated() method, you get all the form current values in $get(fieldName), and then set the value of any other field with $set(fieldName, newValue).

This snippet comes from a bigger premium example CRM: Customer Management System