How do I prevent the last name showing up in my form field?

I have created an email autoresponder and I want to personalize the email to say Dear FIRST NAME, but I do not see a way to choose first name only in the form field. When I build the form, it is requiring both the first name and last name to be filled in, and then when I put NAME in the form field, it puts the first and last name all run together. 

User the following variable: {name:first}

Example:

Dear {name:first}

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