while creating in service account the API key is automatically generated but when I try calling the API service in postman it is asking key and value. Where can I get the those values.
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You will need to change the type authentication in postman to API key. Then copy the value from Appian.
docs.appian.com/.../Web_API_Authentication.html
while create api auth we are only getting api key. how can i get the value?
What do you mean? When you create the API key in Appian, it is displayed once.