Maybe this question already discussed but:
How to weld the ring in sheetmetal?
The joint you show isn't recognised as a suitable joint for a sheet metal Welded Corner. This is a limitation of the SM module.
Suggest you submit an ER for that function.
If you are asking how to insert a "weld bead" into that one part, I don't think you can because you have to have more than one body. However, you can just sketch and extrude the bead profile yourself.
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