I have a PopupBox that I want to change the icon for. Currently it defaults to DotsVertical and I would like to have it as a DotsHorizontal

Figured it out and will leave an answer here in case anyone else comes across this issue. There is a property called ToggleContent

<materialDesign:PopupBox PlacementMode="BottomAndAlignRightEdges" StaysOpen="False">
    <materialDesign:PopupBox.ToggleContent>
        <materialDesign:PackIcon Kind="DotsHorizontal" />
    </materialDesign:PopupBox.ToggleContent>
    <StackPanel>
        <TextBlock Text="Test1" />
        <TextBlock Text="Test2" />
        <TextBlock Text="Test3" />
    </StackPanel>
</materialDesign:PopupBox>