Prepopulate Woo-commerce billing email field from GET parameter

wcf_prepopulate_email.php

/**
 * Pre-populate Woocommerce checkout fields
 */
add_filter( 'woocommerce_checkout_get_value', 'prepopulate_email_field', 10, 2 );

function prepopulate_email_field( $input, $key ) {

	switch ( $key ) {

		case "billing_email":
                        // Modify inf_field_email with your GET paramter from URL.
			if( isset( $_GET['inf_field_email'] ) && ! empty ( $_GET['inf_field_email'] ) ) {
				return $_GET['inf_field_email'];
			}
		break;
	}

	return $input;
}