Getting auth token from on prem Power BI report server

Hi, I have a locally installed Report Server. I have a login user name and password for this server. I need to embed the report published in this server as an iframe in a web application. So, I have to login to the server and get an auth token and pass the token along with the iframe URL. I have the following curl commnd to get the token.

curl --location --request POST 'http://localhost:8800/ReportServer/token' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'grant_type=password' \
--data-urlencode 'username=shef' \
--data-urlencode 'password=test123'

I am getting 401 Unauthorized, when I execute the above. My question is whether my approach is correct or not. Usernamme and password is correct because I am able to login to power bi server with this credential.