It seems that Motorola's Contact app has no support to add offline contacts. I tried installing the APK of Contacts from Motorola and it doesn't show SIM or Phone as an option to save contacts.
However, it does show (in search and normal listing) the contacts which are already stored on the phone or SIM.
So, what you can do is to use a 3rd party contacts app to add contacts on your phone. Then you can continue to use the stock contacts app to access them and dial.
Google contacts is a nice app which closely resembles Moto contacts, but does come with the ability to store contacts on the device.
Original answer and mostly irrelevant: Leaving just because there are comments related to it.
When creating a new contact, you will have the options to choose where to save the contact.
Simply choose Phone or SIM from the Saving to drop-down and you should be good to go. Here's how it looks on my Nokia 7. It is on Android 8.1 as well. I know the Motorola implementation will slightly vary, but it will be overall the same.