How to interact with checkout page in custom php app?

hello,

I have created a custom PHP app, and for that app, I have created a simple form with one textbox and button,

when I enter some text in the textbox then click on submit button, then that text is displayed on the checkout page of the storefront.

I don’t know how to interact with custom and checkout page?

developers, please help me with this.

how can I do this?

App Development in PHP(Laravel/React JS).