If you want to do it as a custom form, put the code to set message properties in the Item_Open event handler of your custom form:

Function Item_Open()
   newDate = dateadd("d",-1,now)
   Item.Subject = "*** Delinquency Comparison *** - as of " & _
               FormatDateTime(newDate,vbShortDate)
   Item.To = "blah, blahblah; lastname, firstname"
   newDate =
   Item.Body = "All:" & vbcrlf & _
       "The report is now available at: " & vbcrlf & _
       "\\networkcomp\drive\public\reporting\" & _
       yyyymmdd(newDate) & "_DelinquencyComparison.xls" & _
       vbcrlf & vbcrlf & "Thank you"
End Function

Function yyyymmdd(anyDate)
   If IsDate(anyDate) Then
       yyyymmdd = Year(anyDate) & _
           IIf(Month(anyDate) < 10, "0", "") & _
           Month(anyDate) & _
           IIf(Day(anyDate) < 10, "0", "") & _
           Day(anyDate)
   End If
End Function

Note the use of a custom function to work around the fact that Format() is not available in VBScript.

You'll need to publish the form in order for the code to run.

You could also use VBA code to generate the message with Application.CreateItem and then set its properties.