Start by navigating to your list of Vendors and click on the Vendor you would like to add more contacts to!
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Once you're on the Vendor page, click "Contacts" and in the top right corner, click "+ Add to Vendor"
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From there, it will pull up an entry form for you to enter the vendor contact information. In the organization section, you can put the Vendor name.
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Another way you can edit/add vendor contacts is here: https://intercom.help/use-kojo/en/articles/6221355-changing-the-vendor-associated-with-a-vendor-contact