Log in to your account
From the menu, choose Embeddable Forms
Locate the form you want to customize and click the Pencil (Edit) Icon
Under Settings, toggle the Form Completion Message to ON (toggle will slide to right)
If the toggle is OFF, a default message will be shown to customers
In the text box, enter the message you want your customers to see
To preview the message you entered, on the right-hand side click Confirmation Preview