The 'y' value double parameter is the price.
Draw.ArrowUp(NinjaScriptBase owner, string tag, bool isAutoScale, int barsAgo, double y, Brush brush)
From the code sample in the help guide:
Draw.ArrowUp(this, "tag1", true, 0, Low[0] - TickSize, Brushes.Red);
I am also including a link to a forum post with helpful resources on getting started with NinjaScript and C#.
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