Hi @Moody,
This cannot be achieved by OOTB SharePoint list features. As a workaround, you can use a combination of Power Apps and SharePoint. As per my knowledge, if you are using a Power Apps form, you can set up a conditional visibility rule. When "New" is selected in the "New or Existing" field, you can make the "Dealer Name" field editable instead of using the lookup functionality.
As I'm not an expert in Power Apps and Power Apps is not supported by Microsoft Q&A, I suggest you go to the dedicated forum here: https://community.powerplatform.com/forums/thread/?groupid=b5652dc6-2c99-4e33-8b6f-45be4a896a40. Experts there will give you professional suggestions. Thanks.
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