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Custom connectors in PowerApps only work with Swagger version 2 definitions.
MTAPI JSON: https://wwwappstestext.tc.gc.ca/Saf-Sec-Sur/13/MTAPI-INT/swagger/docs/v1
SIAPI JSON: https://wwwappstestext.tc.gc.ca/Corp-Serv-Gen/5/SIAPI-ISAPI/swagger/v1v2/swagger.json