Opening Excel file in a SharePoint document library from VSTO with specified credentials

Question

I'm trying to open a workbook from within Excel. The Excel file is located on a SharePoint server, within a Document Library.

I use the following code to open the workbook.

newWorkbook = globalsThisAddInApplication.Workbooks.Open(
                            workbookUrl, Type.Missing, false,
                            Type.Missing, Type.Missing, Type.Missing,
                            Type.Missing, Type.Missing, Type.Missing,
                            Type.Missing, Type.Missing, Type.Missing,
                            Type.Missing, Type.Missing, Type.Missing);

When I do this the user has to enter his credentials (username/password dialog). I'm already storing the user credentials, and I want them to be used when the file is opened.

So how do I pass the credentials to SharePoint?