How can I specify environment variables in Swagger 2?

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I have a Swagger 2 API specification. When I import it to Postman every endpoint has the form:

http:///foo/:bar

In Postman I have a url environment variable. I would like the imported dataset to have endpoints with this form:

{{url}}foo/:bar

How do I modify the Swagger yaml to do this? I have tried adding a host key:

host: "{{url}}"

But that is ignored.