@arturoduranmoreno you would need to use the Oauth 2 override section of the iClient. Here is another community post showing how to use Oauth 2 override section and here is an article on how to use it.
You're override body may be something like this:
{
"grant_type":"password",
"client_credentials":"{{iClient.oauth2.clientId}}",
"client_secret":"{{iClient.oauth2.clientSecret}}"
}