This should work…
import org.sonatype.nexus.capability.CapabilityReference
import org.sonatype.nexus.capability.CapabilityType
import org.sonatype.nexus.internal.capability.DefaultCapabilityReference
import org.sonatype.nexus.internal.capability.DefaultCapabilityRegistry
def capabilityRegistry = container.lookup(DefaultCapabilityRegistry.class.getName())
//Capability specific values/properties
def capabilityType = CapabilityType.capabilityType("webhook.repository")
def capabilityProps = ['names':'asset,component','repository':'maven-releases','secret':'mysecret','url':'http://localhost:1234']
def capabilityNotes = 'Configured through scripting api'
log.info('Capability created as: {}', capabilityRegistry.
add(capabilityType, true, capabilityNotes, capabilityProps).toString()
)