Using the swcbcf_validate_callback_{location}_{field-slug} filter to validate fields on the "Simple Custom Fields for WooCommerce Blocks Checkout" plugin

swcbcf_validate_callback_{location}_{field-slug}.php

<?php
// Filter name: swcbcf_validate_callback_{location}_{field-slug}
// {location} can be contact, address or order
// {field-slug} is the field slug
// This needs to be done at a plugin level, before woocommerce_init priority 10

// In this example we're validating a field that needs to be at least 5 chars long
add_filter( 'swcbcf_validate_callback_' . 'order' . '_' . 'how-did-you-get-to-know', function( $return, $value, $field ) {
	if ( strlen( trim( $value ) ) < 5 ) {
		return 'This field needs to be at least 5 characters long';
	}
}, 10, 3 );