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In order to have the tax code automatically populate as EX, you need to go to the business partner's master data, Business Partner > Business Partner Master Data, pull up the vendor in question, then select the addresses tab. Inside the address tab, click on the Ship To address that pertains to this tax code and set the tax code as from the list that pulls up. In this example, we are using "EX" for the tax code on this ship to address (see highlighted below):
Then Once finished, click the Update button to confirm. From now on when you create a sales document for that customer and using that ship to , address the EX tax code will come onto the document automatically.