Is it possible to assign a value to a custom field once a product is purchased?

Yup! You will want to look at the surecart/purchase_created action:

https://developer.surecart.com/docs/purchase-actions-reference

add_action(
'surecart/purchase_created',
function ( $purchase ) {
$wp_user = $purchase->getUser();
$existing = get_user_meta( $wp_user->ID, 'custom_increment', true );
update_user_meta( $wp_user->ID, 'custom_increment', $existing + 1 );
}
);

This will increment when it is initially purchased. https://developer.surecart.com/docs/purchase-actions-reference

To do a renewal, you will want to use surecart/subscription_renewed

https://developer.surecart.com/docs/subscription-actions-reference

Any other questions, let me know.

Thank you for the example and links to the documentation. This is now working however, the field value seems to increase by 2 instead of 1?

add_action( 'surecart/purchase_created', 'purchaseCreated' );
function purchaseCreated( $purchase, $webhook_data = [] ) {
	$wp_user = $purchase->getUser();
	$boosts = (int) get_field( 'field_65baaaee56f72', 'user_' . $wp_user->ID );
	$boosts++;
	update_field('field_65baaaee56f72', $boosts, 'user_' . $wp_user->ID);
	error_log('Purchase Created #' . $boosts);
};

When I purchase a test order, the value in the field increases by one (as expected) but if I leave it for about 10 seconds, the value in the field increases again by 1.

Is there a way to resolve this issue?

Many thanks.

It may be that the purchase_created hook is firing twice. Sometimes this can happen so that we can run integrations on sites locally, or otherwise unaccessible to webhooks. Can you add this condition at the front? This will ensure it only runs for the webhook context:

add_action( 'surecart/purchase_created', 'purchaseCreated', 10, 2 );
function purchaseCreated( $purchase, $webhook_data = [] ) {
	if ( empty( $webhook_data['id'] ) ) {
		return;
	}
	$wp_user = $purchase->getUser();
	$boosts  = (int) get_field( 'field_65baaaee56f72', 'user_' . $wp_user->ID );
	$boosts++;
	update_field( 'field_65baaaee56f72', $boosts, 'user_' . $wp_user->ID );
	error_log( 'Purchase Created #' . $boosts );
};

I have added this condition however, the $webhook_data array always seems to be empty and causes the rest of the code to fail to run?

Hey smams!

Can you kindly open a support ticket? Our dedicated team will promptly investigate the issue and provide a timely response!

Best,