In the API policy <api-policy.xml>, I'm sending a request to retrieve the authenticationToken

In the API policy <api-policy.xml>, I’m sending a request to retrieve the authenticationToken.

${backend_url}

I’m unable to set the value of the variable ${backend_url} from the terraform script/deployment.yaml, this value is environment specific. Tried many different ways to set this variable but nothing worked. I’m new to APIM, any help?