Assume role without instance profile

// registerManagedInstance checks for activation credentials and performs managed instance registration when present
func registerManagedInstance(log logger.T) (managedInstanceID string, err error) {
    // try to activate the instance with the activation credentials
    publicKey, privateKey, keyType, err := registration.GenerateKeyPair()
    if err != nil {
        return managedInstanceID, fmt.Errorf("error generating signing keys. %v", err)
    }

    // checking write access before registering
    err = registration.UpdateServerInfo("", "", "", privateKey, keyType, "", registration.RegVaultKey)
    if err != nil {
        return managedInstanceID,
            fmt.Errorf("Unable to save registration information. %v\nTry running as sudo/administrator.", err)
    }

    // generate fingerprint
    fingerprint, err := registration.Fingerprint(log)
    if err != nil {
        return managedInstanceID, fmt.Errorf("error generating instance fingerprint. %v", err)
    }

    service := anonauth.NewClient(log, region)
    managedInstanceID, err = service.RegisterManagedInstance(
        activationCode,
        activationID,
        publicKey,
        keyType,
        fingerprint,
    )

    if err != nil {
        return managedInstanceID, fmt.Errorf("error registering the instance with AWS SSM. %v", err)
    }

    err = registration.UpdateServerInfo(managedInstanceID, region, "", privateKey, keyType, "", registration.RegVaultKey)
    if err != nil {
        return managedInstanceID, fmt.Errorf("error persisting the instance registration information. %v", err)
    }

    // saving registration information to the registration file
    reg := map[string]string{
        "ManagedInstanceID": managedInstanceID,
        "Region":            region,
    }

    var regData []byte
    if regData, err = json.Marshal(reg); err != nil {
        return "", fmt.Errorf("Failed to marshal registration info. %v", err)
    }

    if err = ioutil.WriteFile(registrationFile, regData, appconfig.ReadWriteAccess); err != nil {
        return "", fmt.Errorf("Failed to write registration info to file. %v", err)
    }

    return managedInstanceID, nil
}