The only way to send a message on the Lovebox is by using the app. Fortunately, the app is available on iOS and Android! It is easily found by typing "Lovebox" directly in the search bar of the App Store. ☝️
If you have a Lovebox V1 you can use an old tool via this website. ⚠️ This site does not allow you to send messages to the Lovebox Color & Photo and is not really intuitive.