Option available only for zones with multiple IPs allocated. To target a specific IP allocated to your zone, use -ip-ip_address request parameter.

Shell
curl "http://target.site" \
    --proxy brd.superproxy.io:22225 \
    --proxy-user brd-customer-<CUSTOMER_ID>-zone-<ZONE_NAME>-ip-1.1.1.1.1:<ZONE_PASSWORD>