This function adds a default commissionrate when creating affiliate code at checkout.

my-pmpro-generate-affiliate-code.php

<?php   // do not copy this line.
/**
 * This recipe will allow you to set a default commission when using "Automatically create affiliate code?".
 * This setting can be found in your level settings.
 * 
 * For use with Affiliate Add On https://www.paidmembershipspro.com/add-ons/pmpro-lightweight-affiliate-tracking/
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
// change $commissionrate on line 33 as needed.


// Remove the existing function if it exists
remove_action( 'pmpro_after_checkout', 'pmpro_affiliates_generate_affiliate_after_checkout', 10, 2 );

// Add your custom function
function custom_pmpro_affiliates_generate_affiliate_after_checkout( $user_id, $morder ) {
	global $wpdb;

	// Get some info
	$user = get_userdata($user_id);
	$pmpro_level = pmpro_getSpecificMembershipLevelForUser( $user_id, $morder->membership_id );

	// Generate the affiliate after membership checkout
	$pmpro_create_affiliate_level = get_option( 'pmpro_create_affiliate_level_' . $pmpro_level->id );
	$code = pmpro_affiliates_getNewCode();
	if ( ! empty( $pmpro_create_affiliate_level ) ) {
		$days = intval( apply_filters( 'pmproaf_default_cookie_duration', 30, $user_id, $pmpro_level ) );

		// Set the default commission rate to 10%
		$commissionrate = 0.10;

		$sqlQuery = "INSERT INTO $wpdb->pmpro_affiliates (code, name, affiliateuser, trackingcode, cookiedays, enabled, commissionrate) VALUES('" . esc_sql( $code ) . "', '" . esc_sql( $user->display_name ) . "', '" . esc_sql( $user->user_login ) . "', '', $days, '1', $commissionrate)";
		$wpdb->query( $sqlQuery );
	}
}
add_action( 'pmpro_after_checkout', 'custom_pmpro_affiliates_generate_affiliate_after_checkout', 10, 2 );