Try these codes

Option Explicit
Dim swApp As SldWorks.SldWorks
Dim swModel As SldWorks.ModelDoc2
Dim swCustProp As SldWorks.CustomPropertyManager
Sub main()
    Set swApp = Application.SldWorks
    Set swModel = swApp.ActiveDoc
    Set swCustProp = swModel.Extension.CustomPropertyManager("") 'Add config name in the quotes if you want configuration specific property
    swCustProp.Add3 "Description", 30, "Test 123", 1 ' Change property name as needed
End Sub