Setting field to defaul if its a certain value

[Jon]

Hi 

I have a custom field which is a Select List (cascading) type that has several different parent options and each parent has different secondary options. When we add this field to an issue it lets us select the parent and secondary option that we want. but in this case if it is DDGS we can select "None" or "-". the issue is that we often forget to select "-" so wondering if there is a way to make it this by default of the parent is "DDGS". 

I also looked at an automation to update the secondary option if the field was "DDGS" to "DDGS- -" but it wont let me select this field, maybe the field type is not supported. 

Does anyone have any ideas? 

thanks,