Manually tested with Confluent Platform 6.2.0

submit_protobuf_schema_to_confluent_registry_via_rest_interface.sh

#!/bin/bash
# set -x

function schemaPost {
  # Use heredoc to strip out newlines, needed so that post is pure JSON
    cat <<EOF
{
  "schemaType": "PROTOBUF",
  "schema": "${escapedSchema}"
}
EOF
}

valueSchema="`cat test_value_schema0.proto`"
escapedSchema=${valueSchema//\"/\\\"} # escape double quotes
curl -s "http://localhost:8081/subjects/TEST-value/versions" -H "Content-Type: application/vnd.schemaregistry.v1+json" --data @<(cat <<EOF
  $(schemaPost)
EOF
)

This assumes you have a protobuf file test_value_schema0.proto and that you have Confluent Schema Registry running on port 8081.

Marked, thanks Ben, really helpful