Hi,
You can do that purely via a translation override in HikaShop. Just go in the back-office:
1. In Joomla admin, go to Components → HikaShop → Configuration → Languages.
2. Click the edit icon for your current language. In the editor, copy the line with the key for “Add to cart” (probably `ADD_TO_CART`) into the override textarea.
3. Change the translation on the right-hand side to your preferred wording (for example, “Reserve your spot”) and save.
This way, your custom text will persist across updates.
For more details, see the “How to modify translations” documentation :
www.hikashop.com/download/languages.html#modify