Following works, change as per your url, client_id, client_secret, redirect_url
{
"name": "okta",
"enable_social_login": 1,
"social_login_provider": "Custom",
"client_id": "yourclientid",
"provider_name": "okta",
"client_secret": "****************************************************************",
"icon": "/files/okta_sso.png",
"base_url": "https://trial-1234567.okta.com/oauth2/default",
"authorize_url": "/v1/authorize",
"access_token_url": "/v1/token",
"redirect_url": "https://erp.example.com/api/method/frappe.integrations.oauth2_logins.custom/okta",
"api_endpoint": "https://trial-1234567.okta.com/oauth2/default/v1/userinfo",
"auth_url_data": "{\"scope\":\"openid email profile\",\"response_type\":\"code\"}"
}