Hello there Juan!
Will here from Shopify to help out with your store and gathering additional information during checkout.
Currently, our system is limited on what we can actually modify in the checkout. That being said, it doesn't mean we can't get this before-hand!
This may vary depending on your exact Theme but, you can use a this guide within our Theme development part of the help section to input some additional fields. The example used is for a drop down but, you can also look at adding a cart note (varies by theme)
Either one of these may work if both options are available. The indicator not included on every Theme, and may not work if it is not available.
I hope that helps you! If you need further assistance, definitely let us know. Thanks for taking the time to reach out to us.
Guru @ Shopify