ok, eu fiz essa implementacao
public async Task SaveWebhookUrl(Guid chavePix, string webhoookUrl)
{
dynamic efi = new EfiPay(
appSettings.Payment.ClientId,
appSettings.Payment.ClientSecret,
appSettings.Payment.Sandbox,
appSettings.Payment.Certificate
);
var headers = "{\"x-skip-mtls-checking\": \"true\"}";
var param = new
{
chave = chavePix.ToString()
};
var body = new
{
webhookUrl = webhoookUrl
};
await Task.FromResult(efi.PixConfigWebhook(param, body, headers));
}