You can achieve horizontal navigation by hiding the side nav and adding links to the NavbarLinks array.
$Page1 = New-UDPage -Name "Page1" -Content {}
$Page2 = New-UDPage -Name "Page2" -Content {}
$Navigation = New-UDSideNav -None
$Dashboard = New-UDDashboard -Title "Test" -Pages @($Page1, $Page2) -Navigation $Navigation -NavbarLinks @(
New-UDLink -Text "Page 1" -Url "/page1"
New-UDLink -Text "Page 2" -Url "/page2"
)
Start-UDDashboard -Dashboard $Dashboard -Port 10000