`relabel_configs` in river format

Hi, I have configured grafana-agent for my k8s cluster. Cluster data are working great. Now I want agent to scrape a custom prometheus endpoint for my application. The discovery_relabel works great, now I need to rewrite the scrape endpoint port. From documentation it seems I need to set a block relabel_configs, but I receive an error “unrecognized block name”, I guess I put the block in the bad context: someone could tell me where to put my block? Thank you. Here my current snippet:

    discovery.relabel "cultsprod_front_puma_pods" {
      targets = discovery.kubernetes.pods.targets  // Gets all pods
      rule {
        source_labels = ["__meta_kubernetes_pod_name"]
        regex = "cultsprod-front"
        action = "keep"
    prometheus.scrape "cultsprod_front" {
      targets = discovery.relabel.cultsprod_front_puma_pods.output
      forward_to = [prometheus.remote_write.metrics_service.receiver]
      scrape_configs {
        relabel_configs {
          source_labels = ["__address__"]
          action = "replace"
          regex = "([^:]+):.*"
          replacement = "$1:9393"
          target_label = "__address__"

To be more precise, if I remove the scrape_configs block, it’s correctly parsed.
Thanks again.