You will need iOS 10.3 or watchOS 3.2 to use Siri with Debit & Credit. To create meaningful transactions, you need to be using locations for your payees.
How to Use Siri With Debit & Credit
Since iOS 10.3 Siri knows how to pay bills. Debit & Credit is using that for creating expenses since these two actions (paying a bill and creating an expense) have many things in common. However, this comes with many limitations since the app cannot adjust Siri's behaviour.
First of all you need to enable Siri for Debit & Credit. Open the app settings, select "Advanced Settings" and then "Siri".
To start using Siri, you need to ask her to pay a bill. The best way to do that is to say "Siri, pay a bill in Debit & Credit". You can also specify expense amount right in the original command by saying: "Pay a bill for $100 in Debit & Credit".
Once you do that, Siri will look for nearby payees. It will either use a payee that is close enough to you or will provide multiple payees to select from. If you don't have any payee locations saved in the app, your transaction will be created without a payee.
Once a payee is selected, by default Siri will also choose the most appropriate category and account for that payee. You will be asked to confirm a transaction before it is saved.
Alternatively, you can try specifying payee and account name right in your Siri command. For instance, you could say "Pay my ALDI bill" or "Pay my ALDI bill with my Visa account".
Please note that when presenting accounts for selection Siri will put account type in front of account name, e.g. "Credit Visa" for "Visa" account. This is related to a bug in iOS 10.3, you can ignore account type.
Settings for Siri
There are some advanced settings related to Siri. You can find them in the app settings.
Select Last Used Category - when creating a transaction, payee's last used category will be automatically selected.
Default Account - you can optionally designate an account to be used for Siri transactions.
Match Accounts With Payees - only accounts that you have previously used with a payee will be provided for selection.
Siri on Apple Watch
You can create expenses on the Apple Watch in the same manner as in iOS. However, you will need to open the watch app at least once to allow the app to download necessary records.
If You Don't Use Siri in English
Unfortunately, Siri fails to understand many english app names (including Debit & Credit) if you use Siri in a different language. In this case you need to say "Pay a bill for 100" to Siri and then confirm that you want to use Debit & Credit.