If you set the Title property of the document, when you choose Save As, that is the document name that will be used. You can also set the default save location. In VBA
Set doc = ActiveDocument
sTitle = doc.BuiltInDocumentProperties("Title").Value
doc.BuiltInDocumentProperties("Title").Value = "A different title"
However, this only works on the second (and later) save attempt(s). The first attempt will always use the title from the template, if any, or content from the first line of the document if none. See the end of this answer for a better solution.
Note, however, that you must make some change to the document before Save As for the new title to take effect.
Sub SetSummaryInfo()
Dim dp As Object
Dim sTitle As String
If Documents.Count > 0 Then
Set dp = Dialogs(wdDialogFileSummaryInfo)
' Retrieve value of "Title" into a variable.
sTitle = dp.Title
' Set "Title" to a new value.
dp.Title = "My Title"
' Set the value without showing the dialog.
dp.Execute
' Save the changes
'ActiveDocument.Save
End If
End Sub
As remarked by HCL in C#, you can set the default filename (for the dialog only) using this code:
dynamic dialog = wordApp.Dialogs[WdWordDialog.wdDialogFileSummaryInfo];
dialog.Title = "MyTitle";
dialog.Execute();
This opens the standard "Save As" dialog, sets the default filename (not what you'd expect from a 'Title' property but that's what it does), and opens the dialog.