I see. You are right, there is no explicit tracing function in ParaView. A couple options come to mind:
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There are point and cell selection tools you can activate by clicking the small icons just above the 3D render view. You probably would want to use the “Interactive Select Points On” to select points in the region you want. Then click the points you want to treat as stream tracers seeds and add an Extract Selection filter to pull these points into a separate data set that you can use as the “Seed Source” in the Stream Tracer With Custom Source filter.
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Add a Poly Line Source and use it as the “Seed Source”. The Poly Line Source lets you draw a polygonal region, though the points are in 3D, so it’s a little less convenient than say drawing on a single 2D image. But you can interactively move the points in the source and these will be treated as seed points in the stream tracer.