Thank you for your quick response! 
I tried all 3 options for LinkingDirection (ForwardsOnly, BackwardsOnly, Either). Unfortunately I didn’t get the desired behavior.
Below I’ve created what I think is close to a minimum-working example to illustrate what I’ve got and the desired use-case. When run, this is the image I initially get when I try to create a link from Operand-2 to Operator by clicking and dragging out FROM Operator’s lower-left rectangle (highlighted in pink).
After I create that link, this is what I get, so the new link is just backwards (for this use-case). Operand-2 should be flowing into Operator, not vice-versa.
I realize this isn’t a typical use-case, and overriding certain methods of the LinkingTool might be appropriate, I just couldn’t figure that out on my own. I saw documentation for overriding link validation, but not link-creation.
Here is the code-behind:
public MainWindow()
{
InitializeComponent();
diagram.Model = new GraphLinksModel<GraphLinksModelNodeData, String, String, GraphLinksModelLinkData<string, string>>
{
Modifiable = true,
NodesSource = new List<GraphLinksModelNodeData>()
{
new GraphLinksModelNodeData(){Text=“Operator”, Key=“Operator”, Category=“OperatorRenderKey”},
new GraphLinksModelNodeData(){Text=“Operand-1”, Key=“Operand-1”, Category=“OperatorRenderKey”},
new GraphLinksModelNodeData(){Text=“Operand-2”, Key=“Operand-2”, Category=“OperatorRenderKey”}
},
LinksSource = new List<GraphLinksModelLinkData<string, string>>()
{
new GraphLinksModelLinkData<string, string>(){From=“Operand-1”, FromPort=“Output”, To=“Operator”, ToPort = “Input2”}
}
};
//diagram.LinkingTool.Direction = Northwoods.GoXam.Tool.LinkingDirection.ForwardsOnly;
}
And here is the XAML:
<go:NodePanel Cursor=“Hand” Sizing=“Auto”>
<go:NodeShape Opacity=“1” Stroke=“Pink” StrokeThickness=“1” go:NodePanel.Figure=“StopSign”/>
<Grid.ColumnDefinitions>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
</Grid.RowDefinitions>
</go:NodePanel>
</Window.Resources>
