You have a little misunderstanding of terminology here. DocProperty fields
don't store anything: they display the contents of a custom or built-in
document property (which you get to through File > Properties). It's the
property that stores the value, not the field.

So what you need your form to do is read, display, and set the document
property.

If it's a built-in document property use

txtBox = activedocument.BuiltinDocumentProperties("Name")

activedocument.BuiltinDocumentProperties("Name") = txtbox

if it's a custom property, use .CustomDocumentProperties("Name") ..

After changing the property, you need to update fields in the document to
display the new value.