how to add dll file to Windows Application Packaging Project

Hi there, I have a c++ win32 application, and I wan't to pack it to msix, i found this tutorial. but my project has some dll file that is other project's compiled product.

the solution like this:

when I add the DLL1's project reference visio studio prevent me. it is because that the DLL1 project doesn't have exe file.

so, how do I add these dll files in Windows Application Packaging Project.