Insert array to mysql, but add other mysql-based value to each new row before insert

I have a table 'images' like this:

In one mysql query, I want to insert multiple rows. But, most importantly, I want to populate the next SortOrder for each row(starting at the last one for that ProductId).

For example, I would love to be able to do:

const values = [
    [2,'source.com/456546', @max_sort_order := @max_sort_order + 1],
    [2,'source.com/237675', @max_sort_order := @max_sort_order + 1]
]

const query = "SET @max_sort_order := (SELECT COALESCE(MAX( SortOrder ), 1) FROM images i WHERE ProductId = 2);INSERT INTO images (ProductId, Source, SortOrder) VALUES ?"

connection.query({sql: query, timeout: 40000, values: [values]...

Which would ideally result in the following new rows:

But I can't put that @max_sort_order variable in the values, due to it being prepared values(I think).

Any idea on how to be go about this?

The simplest solution is secondary autoincremented column in MyISAM table:

CREATE TABLE images (
    ProductId INT,
    SortOrder INT AUTO_INCREMENT,
    Source VARCHAR(160),
    PRIMARY KEY (ProductId, SortOrder)
    ) ENGINE = MyISAM; 
INSERT INTO images VALUES 
(1, 1,  'source.com/234/'),
(1, 2,  'source.com/675/'),
(2, 1,  'source.com/7698/'),
(2, 2,  'source.com/678/'),
(2, 3,  'source.com/7456/');
SELECT * FROM images;

INSERT INTO images (ProductId, Source) VALUES
(2,'source.com/456546'),
(2,'source.com/237675');
SELECT * FROM images;

fiddle