Salesforce picklist

Hello, I am creating forms that update a picklist field in salesforce. The problem is, the form currently completely overwrites the field, instead of adding. So if a user has the picklist options “Item 1, Item 2”, if I submit a form to add “Item 3”, the form submission overwrites “Item 1, Item2” and it only shows “Item 3”. I need the value to be ADDED to the picklist