mysql> CREATE TABLE master (master_id INT AUTO_INCREMENT PRIMARY KEY,
    ->                      master_value VARCHAR(255));
Query OK, 0 rows affected (0.34 sec)

mysql> CREATE TABLE slave (slave_id INT AUTO_INCREMENT PRIMARY KEY,
    ->                     slave_value VARCHAR(255),
    ->                     master_id INT,
    ->                     CONSTRAINT fk_master FOREIGN KEY (master_id) REFERENCES master (master_id));
Query OK, 0 rows affected (0.79 sec)

mysql> CREATE TRIGGER tr_ai_master_fill_slave
    -> AFTER INSERT
    -> ON master
    -> FOR EACH ROW
    -> INSERT INTO slave (slave_value, master_id)
    -> SELECT jsontable.slave_value, NEW.master_id
    -> FROM ( SELECT CAST(@master_tmp_var AS JSON) data ) source
    -> CROSS JOIN JSON_TABLE( source.data,
    ->                        '$.slave_value[*]' COLUMNS ( slave_value VARCHAR(255) PATH '$' ) ) jsontable;
Query OK, 0 rows affected (0.10 sec)

mysql> LOAD DATA INFILE 'C:\\ProgramData\\MySQL\\MySQL Server 8.0\\Uploads\\source.json'
    -> INTO TABLE master (@master_tmp_var)
    -> SET master_value = JSON_UNQUOTE(JSON_EXTRACT(@master_tmp_var, '$.master_value'));
Query OK, 2 rows affected (0.06 sec)
Records: 2  Deleted: 0  Skipped: 0  Warnings: 0

mysql> SELECT * FROM master;
+-----------+----------------+
| master_id | master_value   |
+-----------+----------------+
|         1 | master value 1 |
|         2 | master value 2 |
+-----------+----------------+
2 rows in set (0.00 sec)

mysql> SELECT * FROM slave;
+----------+---------------+-----------+
| slave_id | slave_value   | master_id |
+----------+---------------+-----------+
|        1 | slave value 1 |         1 |
|        2 | slave value 2 |         1 |
|        3 | slave value 3 |         1 |
|        4 | slave value 4 |         2 |
|        5 | slave value 5 |         2 |
+----------+---------------+-----------+
5 rows in set (0.00 sec)

mysql>