If the option is activated in the 'Advanced settings' of the 'Company settings', you will find this type of expense form when creating a new expense in your online account:
You can fill in all of the relevant details before clicking on 'Save and submit'.
At that point, the purchase order will be sent to your approver - who also approves your expenses - and it will get the status 'approved PO':
Whenever you have actually made the pre-approved purchase, you can link the receipt to this purchase order by creating the expense via the receipt upload. After saving the created expense, our application will recognize the information and propose the link.
When reopening the expense an extra button will be visible in your created expense to create the link:
If the proposal is correct you can confirm by hoovering over the correct purchase order and clicking on the 'V-button' that appears.