Code (C++)

#include "esp_wifi.h" (part of IDF (C) component in Arduino ESP32)

    wifi_country_t wifi_country;
    char code[3] = "US";
    //char *code = "US";
    wifi_country.cc = &code;//char[3]country code string 

Code (C++)

/** @brief Structure describing WiFi country-based regional restrictions. */
typedef struct {
    char                  cc[3];   /**< country code string */
    uint8_t               schan;   /**< start channel */
    uint8_t               nchan;   /**< total channel number */
    int8_t                max_tx_power;   /**< This field is used for getting WiFi maximum transmitting power, call esp_wifi_set_max_tx_power to set the maximum transmitting power. */
    wifi_country_policy_t policy;  /**< country policy */
} wifi_country_t; 

Code (C++)

void setCountry() {
    ////before esp_wifi_start
    esp_err_t err;
    wifi_country_policy_t wifi_country_policy;
    //wifi_country_policy = WIFI_COUNTRY_POLICY_AUTO;//WIFI_COUNTRY_POLICY_MANUAL
    //wifi_country_t wifi_country;
    //char code[3] = "US";
    ////char *code = "US";
    //wifi_country.cc = code;//char[3]country code string
    //wifi_country.schan = 0;//uint8_t start channel
    //wifi_country.nchan = 13;//uint8_t total channel number
    ////wifi_country.max_tx_power = esp_wifi_set_max_tx_power;//int8_t esp_wifi_set_max_tx_power to set the maximum transmitting power

    wifi_country_t wifi_country = { .cc = "US", .schan = 1, .nchan = 11, .policy = WIFI_COUNTRY_POLICY_AUTO };

    err = esp_wifi_set_country(&wifi_country);
}