Can a textbox be added in the middle of a sentence?

Like this?

I {textbox field} certify that I am an adult with full legal authority to enter into this contract for myself and/or on behalf of the guest(s) I select below.

If you mean if you can reference a textbox or some other field, then yes, you can do that in the Text field, all you would need is the correct field tag.

You can find it out by following the steps in this guide: How to find Field IDs & Names

If you have any difficulties do let us know the form and the fields in question and we would be happy to assist with this.