How to use if condition in powerapp for populating dropdown which depends on selection of a control

I am building a powerapp where I have a radiobutton list , if any item of the radiobuttonlist is selected then dropdown will populate accordingly else it should show a default value such as Select a Product. I want to use if conditon for Items Property of dropdowncontrol this e.g.

Items = if(IsBlank(Radio.Selected.Value),DropDown.Selected.Value = "Select Product", Filter(ProductDemands, ProductValue = Radio.Selected.Value))

---

where ProductDemands is name of entity ProducValue is column name

I am unable to acheive the same. I am getting syntax errors and warnings for the same.