The official features don’t at present support setting user codes, but there are two good community created options that do. In order to use these you have to use what is called “custom code.” That’s pretty straightforward, it just means that you copy the author’s code and paste it into your own account. Here’s the FAQ that describes that process (this is a clickable link)
If after reading that you decide that’s something you’d like to move forward with, the next question is how do you find custom code to copy?
The easiest way is usually to check the quick browse lists in the community – created wiki. These are organized into sections for smart apps and device type handlers and project reports. In this case you would be looking for a smartapp. So look for the list called “locks” in the smartapp section:
You will find a number of different choices there. The most popular free smartapp is probably “lock manager” and it has many different features. There’s also a paid app from @Rboy which many people like.