You will need to create a PI Mapping first for the Windows users. Then move the Windows options to the top of the PI SDK protocol order so it will be tried first. In the PI SDK code, you should not need to explicitly specify the user. PI SDK internally will detect what the current Windows user running the application is and present these credentials to the PI Data Archive.
I just want to remind you that PI AF SDK is our recommended SDK for new development projects. If you have WIS properly configured, you will able to use the privileges of the domain account running your application to access the PI Data Archive, the same way that PI SDK does internally as explained by Barry
If this is not a new project, I would consider this opportunity to migrate to PI AF SDK since this is the product that our development team is currently working on and adding new enhancements.