Add Webster Dictionary to my iPad Pro
Unfortunately, what you are trying to accompanist is not possible due to nature of how iOS works.

The list of dictionaries available under Settings app → General → Dictionary is curated by Apple, and can not be altered/added-to by end users.

If you download a dictionary app from the App Store, it does not, and cannot be automatically integrated into the iOS operating system and made available system wide.

To use the Webster dictionary, in your case, you'll have to manually type/copy the word into the app and determine the meaning.