As Shopify has released new checkout extensibility you can setup our app OTP block where you want to add in your thank you page,

To add OTP verification block on thank you page please follow below steps,

1) Open your store settings and click on checkout menu, there you will find checkout customize option.

2) Open customize settings and select "Thank you" page from upper drop-down and find "Add Block" option (Do not forget to publish this page before save).

3)  Here you will get list of blocks which is installed in your store, you need to find "Cashify COD Confirmation" app block and click on it.

4) Just click back button and it'll be added under Order detail section, click on "Save" 

Setup has been completed, also make sure you have enabled the "OTP" mode from app settings in our app.