Try this routine:
Public Sub updateCustomDocumentProperty(strPropertyName As String, _
varValue As Variant, docType As Office.MsoDocProperties)
On Error Resume Next
ActiveWorkbook.CustomDocumentProperties(strPropertyName).Value = varValue
If Err.Number > 0 Then
ActiveWorkbook.CustomDocumentProperties.Add _
Name:=strPropertyName, _
LinkToContent:=False, _
Type:=docType, _
Value:=varValue
End If
End Sub
Edit: Usage Examples
Five years later and the 'official' documentation is still a mess on this... I figured I'd add some examples of usage:
Set Custom Properties
Sub test_setProperties()
updateCustomDocumentProperty "my_API_Token", "AbCd1234", msoPropertyTypeString
updateCustomDocumentProperty "my_API_Token_Expiry", #1/31/2019#, msoPropertyTypeDate
End Sub
Get Custom Properties
Sub test_getProperties()
MsgBox ActiveWorkbook.CustomDocumentProperties("my_API_Token") & vbLf _
& ActiveWorkbook.CustomDocumentProperties("my_API_Token_Expiry")
End Sub
List All Custom Properties
Sub listCustomProps()
Dim prop As DocumentProperty
For Each prop In ActiveWorkbook.CustomDocumentProperties
Debug.Print prop.Name & " = " & prop.Value & " (" & Choose(prop.Type, _
"msoPropertyTypeNumber", "msoPropertyTypeBoolean", "msoPropertyTypeDate", _
"msoPropertyTypeString", "msoPropertyTypeFloat") & ")"
Next prop
End Sub
Delete Custom Properties
Sub deleteCustomProps()
ActiveWorkbook.CustomDocumentProperties("my_API_Token").Delete
ActiveWorkbook.CustomDocumentProperties("my_API_Token_Expiry").Delete
End Sub