Defaulting a value using custom pll for a item which is a LOV in the form

Hello All,
I am working on a custom pll requirement in Customer standard form, In addresses block. The requirement is like:

Every time when a new customer site is created or updated the address (Address in US) will be validated with a third party application to check if it is a valid USPS address. In some cases for vanity cities it will return a different city name which holds the post office for the vanity city. For Example Devon CT is a vanity city and when we validate the address using the third party application it will return the city name as MILFORD CT instead.