I am unable to generate refresh token using the following data

I am using offline access type as per the documentations but still unable to get refresh token. 

Authorization URL ::  https://accounts.zoho.in/oauth/v2/auth?response_type=code&client_id=1000.DUCAO1EBE5DK48197WYXES8UA3****&access_type=offline&scope=ZohoBooks.fullaccess.all&redirect_uri=https://storehippo.com/oauth&state=zoho

1. 
   
2. request: {
   
3.   "url": "https://accounts.zoho.in/oauth/v2/token?client_id=1000.DUCAO1EBE5DK48197WYXES8UA3FB7V&client_secret=******b02adb593e48ff8****54d6906576b******&redirect_uri=https://storehippo.com/oauth&code=1000.805b3812f1eab3d5278b90ce58c08ae7.2123ba93a376fc9842836fd40********&grant_type=authorization_code",
   
4.   "method": "post",
   
5.   "json": true,
   
6.   "resolveWithFullResponse": true
   
7. }
   

1. 
   
2. response : {
   
3.   "access_token": "1000.4538ac6654ee8f5ef0bc554db5e474fc.d918d24a9f911f******10c2b7******",
   
4.   "expires_in_sec": 3600,
   
5.   "api_domain": "https://www.zohoapis.in",
   
6.   "token_type": "Bearer",
   
7.   "expires_in": 3600000
   
8. }
   

Please specify the process how can i get refresh token. Have any changes are made in the API auth flow which is not updated in the documents?