API M Policy error : Microsoft.WindowsAzure.ApiManagement.Proxy: The message body is not a valid JSON. Unexpected character encountered while parsing the Json value

Hello , I am facing an issue.

The GET request to the, https://login.microsoftonline.com/{tenantId}/oauth2/v2.0/token.

in the APIM Policy <Send-Request> block.

Returns Exception: "Unexpected character encountered while parsing value: <. Path '', line 0, position 0."

My Policy:

<send-request mode="new" response-variable-name="bearerToken" timeout="20" ignore-error="false">
      <set-url>https://login.microsoftonline.com/{{aad-tenant}}/oauth2/v2.0/token</set-url>
      <set-method>GET</set-method>
      <set-header name="Content-Type" exists-action="override">
          <value>application/x-www-form-urlencoded</value>
      </set-header>
      <set-body>@{
          return "client_id=68b5d0d9-xxxx-xxxx-xxxx-b921efcfad9b&scope=api://xxxxxxx-c2f1-xxxxx-xxxx-08e09cf10ca4/.default&client_secret=xxxxx~IQ.-uN9EAKtyoWPk3Rm6v-xxxx.xxxxxx&grant_type=client_credentials";
            }</set-body>
</send-request>
<set-header name="Authorization" exists-action="override">
    <value>@("Bearer " + (String)((IResponse)context.Variables["bearerToken"]).Body.As<JObject>()["access_token"])</value>
</set-header>

Also, in the Docs here suggest POST request for the above URL, but POST request results in a bad request error. A GET request to the same URL results 200 OK.
In POSTMAN, Both POST and GET results in 200 OK.
Not Sure what is wrong. But the Main problem is to extract the access_token from the response body which is why i am racing this issue. Please Assist.

PS: I have read the similar Question posted Here in qna community which is not helping me, So please do not redirect me to look for the solution there. "even posted a comment for help there ",

Many Thanks.