How to select the first point of boundary for stitch mesh by "bridge mesh"?

Hi all,

In mesh tool, when I using "stitch into mesh" by "bridge mesh"

the software recommends to "select the first point on the boundary", but I don't know which element to select. I tried to extract boundary of mesh and picked this boundary, but nothing worked as picture below:

Could you please guide me to fix this issue? 

Thank you so much.

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Parker

MicroStation CONNECT Edition - Version 10.16.02.34