'How to Test a WSDL API from a custom c# services?

I'm a newbie in C# and was endorsed with an app with no documentation or anything. I was wondering if there is a way I can test a WSDL API from a custom built C# services? I just want to make sure that the API is working and that the app was not the one causing troubles. I tried postman but it returns me this whenever I tried to access the API.

(Postman Result)

I was hoping someone can help me with this. I tried several tutorials online and it doesn't work. Thank you



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