Adding Product Bundle and variation child to Order/Cart using add_bundle_to_order()

I'm trying to add a Product Bundle to an Order (or a Cart, add_bundle_to_cart uses the same args). The Bundle has several child products, all of which have variations. The args I need to be providing according to the documentation are:

$args = array(
    81 => array(
        'product_id' => 1543,
        'quantity' => 3,
    ),
    84 => array(
        'product_id' => 1386,
        'quantity' => 1,
        'variation_id' => 3535,
        'attributes' => array(
            'attribute_pa_attribute-1' => 'value-1b',
            'attribute_pa_attribute-2' => 'value-2b',
        ),

    )
);

Where the second bundled product (84) is a variable product. My issue: I cannot verify whether the values I am providing for the 'attributes' array are correct.

My attribute is named 'Servings' and there are two options - '2 People' and '4 People'. I am not sure how to format the attribute name to replace 'attribute_pa_attribute-1'. I have tried 'servings', 'attribute_pa_servings', etc. If I return the variation information for the product it gives me: 'attribute_servings' as the name and '2 People' as the value. But that doesn't seem to work either.

I am not getting any errors returned in debug.log, and I cannot find any examples of someone using this in the land of Google.

If anyone has any experience with this, I would greatly appreciate a tip as to what I am doing wrong.

Many thanks.