Hi,
If you have only one characteristic in your product, then the default value is automatically selected.
One solution is to create an additional variant you'll call "select an option" with a quantity of 0 or a sale end date in the past or a sale start date in the future and select it as default.
That way, you'll not have the add to cart by default, and you'll have some text that you can change with a translation override to say something else like "add to cart button will appear after you select something".
www.hikashop.com/download/languages.html#modify