When you want to save data to the sales_order from the quote table or before place an order you want to save data to the sales order table you can achieve this via the event observer approach.
You can save custom field value to the Order table before the order placed. The given event will also useful to copy quote data to the sales order table.
Event Name: sales_model_service_quote_submit_before
You can find the definition for the event from the Quote Module.
vendor/magento/module-quote/Model/QuoteManagement.php Continue reading “How to use sales_model_service_quote_submit_before event Magento 2?”