Validate From: field in Outlook custom form

I have created an Outlook custom form that contains the From: (i.e., [From]) field as one of the form fields.  Basically, I just want to make sure that this field is not left blank, so I need to use validation.  Does anyone know a basic validation syntax I can use within the validation formula?
CecilAdminAsked:
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Richard QuadlingSenior Software DeveloperCommented:
Can you try...

<> ''    (single quotes)
or
<> ""  (double quotes)

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David LeeCommented:
Hi, CecilAdmin.

Can't you use the "A value is required for this field" option?
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CecilAdminAuthor Commented:
^No, it is greyed-out.
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CecilAdminAuthor Commented:
<> ""  (double quotes) works
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