You need to create the custom capability with the corresponding commands (see below).
When you create it, an ID is generated automatically, which you need to include in your device profile.
//to create capability
smartthings capabilities:create //-i capability.yaml
//Note: if we don't use a file as input, the assistant to create the capability is executed
//to create its presentation
smartthings capabilities:presentation:create [ID] [VERSION]
I suggest you take a look at this sample using custom capabilities:
Also, here’s an interesting discussion about it: