I have had problems that i need to do the same thing on many items.
Like this problem, I also the shop to warn and tell customer that he needs to choose. But don't want it to be set as default on some size.
In my case i had 100products, that had different size values.
Some had 80mm,90mm,100mm. Some had 40mm,50mm,70mm. etc.
All my size values go under my optionname "Size"
I made a new size value named "------" (you can choose what you want, this is a pretty good one so try it out first before you change to some text)
I took any product out of the blue that needed this "need to make choice before add to cart" decision.
The product i choose in my shop had for example productnumber #131.
I added this size value "------" to this product.
Now #131 had 4 sizevalues to choose from -> 40mm,50mm,70mm and ------.
I put the "------" as display only and marked as default.
Now go to "Option Values" section.
At the bottom you have this "Copy Option Name/Value to Products with existing Option Name ..."
I did my setting like this.
Option name to add: Size
Option Value to add: ------ [Size]
Default New Attribute Values from Product ID# or leave blank for no default values: 131 (since I have made the desired settings for my "------" size value and want to use this as a template/pattern for adding this values to all my products that has size values.
Option name to add to: Size
How should existing product attributes should be handled?: Mark the radiobutton Update existing attributes with new settings/prices
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You're done! It's now updated all your products that you have added the attribute Size to. It's added ------ value to them all and added it as display only and marked it as default. So the shop will complain and ask customer to make a choice :clap: