mysql> create table t1(id serial primary key, val int);
Query OK, 0 rows affected (0.35 sec)

mysql> create table t2(id serial primary key, val int, t1_id bigint unsigned not null, foreign key idx (t1_id) references t1(id));
Query OK, 0 rows affected (0.21 sec)

mysql> create trigger tr after insert on t1 for each row insert into t2 (val, t1_id) select new.val*3, new.id;
Query OK, 0 rows affected (0.26 sec)

mysql> insert into t1(val) select 1 union select 5;
Query OK, 2 rows affected (0.22 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t1;
+----+------+
| id | val  |
+----+------+
|  1 |    1 |
|  2 |    5 |
+----+------+
2 rows in set (0.03 sec)

mysql> select * from t2;
+----+------+-------+
| id | val  | t1_id |
+----+------+-------+
|  1 |    3 |     1 |
|  2 |   15 |     2 |
+----+------+-------+
2 rows in set (0.00 sec)