By default, Shopify does not support having a confirmation page appear after a user submits a form.
To add this functionality, you may want to check out some of these form apps. However, please note that because these apps are developed by third parties, we're generally not able to provide any guidance on which would be the best fit for you or assistance with installation or technical issues. You'd need to contact the app developer directly with any questions or support required.
However, you can change the confirmation message that is displayed after someone submits a form.
To update the confirmation message, just follow these steps:
Step 1: From your Shopify Admin, head into Sales Channels > Online Store > Themes.
Step 2: Under the Actions menu, select Edit languages.
Step 3: In the Language Files, you can edit the confirmation message for: the newsletter form, contact form, and notify form.
Newsletter form confirmation message
Under General > Newsletter form > Success text:
Contact form confirmation message
Under Contact > Form > Post Success:
Notify form confirmation message
Under Products > Notify form > Post success: