App metafield in checkout extension

[prakashVt]

I want to access metafields in my checkout extension but I'm not sure which metafield is correct to use. I'm confused about app owned metafield, metafield definition, and metaobject definitions

Please note that I already added the code in checkout extension as below 

[[extensions.metafields]]
namespace = "testimonial"
key = "author"

And in my js file I'm using this 

  const metafieldNamespace = "testimonial";
  const metafieldKey = "author";
  
 
  const appMetafields = useMetafield({
    namespace: metafieldNamespace,
    key: metafieldKey,
  });

 console.log(appMetafields, "appMetafields");