This is an example USDA result from creating an Xform and adding three References to it. The first referenced target prim is inside the current file with the prim path /World/intern_target. The second referenced target prim is in the file C:/path/to/file.usd with the prim path /World/some/target and the third is the default prim in the file C:/path/to/other/file.usd.

#usda 1.0
(
    defaultPrim = "World"
)

def Xform "World"
{
    def Xform "ref_prim" (
        prepend references = [
            </World/intern_target>,
            @C:/path/to/file.usd@</World/some/target>,
            @C:/path/to/other/file.usd@
        ]
    )
    {
    }

    def Xform "intern_target"
    {
    }
}