Sample Code

package main

import (
    "bytes"
    "context"
    "encoding/json"
    "github.com/gagliardetto/solana-go"
    "github.com/gagliardetto/solana-go/programs/system"
    "github.com/gagliardetto/solana-go/rpc"
    "io"
    "net/http"
    "testing"
)

type Payload struct {
    Transaction TransactionMessage `json:"transaction"`
}

type TransactionMessage struct {
    Content string `json:"content"`
}

func TestSendTx(T *testing.T) {
    cli := http.Client{}
    privateKey := solana.MustPrivateKeyFromBase58("")
    rpcClient := rpc.New("")
    bh, _ := rpcClient.GetLatestBlockhash(context.TODO(), rpc.CommitmentFinalized)
    txBuilder := solana.NewTransactionBuilder()
    txBuilder.SetRecentBlockHash(bh.Value.Blockhash)
    txBuilder.AddInstruction(system.NewTransferInstruction(1000000, privateKey.PublicKey(), solana.MustPublicKeyFromBase58("nextBLoCkPMgmG8ZgJtABeScP35qLa2AMCNKntAP7Xc")).Build())
    tx, _ := txBuilder.Build()
    tx.Sign(func(key solana.PublicKey) *solana.PrivateKey {
        return &privateKey
    })
    tx64Str := tx.MustToBase64()
    txMsg := TransactionMessage{
        Content: tx64Str,
    }
    p := Payload{
        Transaction: txMsg,
    }
    t, _ := json.Marshal(p)

    req, err := http.NewRequest("POST", "https://fra.nextblock.io/api/v2/submit", bytes.NewBuffer(t))
    req.Header.Add("Content-Type", "application/json")
    req.Header.Add("authorization", "")
    if err != nil {
        T.Error(err)
        return
    }
    do, err := cli.Do(req)
    if err != nil {
        T.Error(err)
        return
    }
    defer do.Body.Close()
    b, _ := io.ReadAll(do.Body)
    T.Log(string(b))
    if do.StatusCode == 200 {
        T.Log("Sent transaction with signature ", tx.Signatures[0].String())
    }
}

CURL Example

curl -XPOST -H 'authorization: api key' -H 'Content-Type: application/json' 'https://fra.nextblock.io/api/v2/submit' -d '{ "transaction": {"content": "AjF+B...BCQ=="} }'