You can create SOA partitions with MyST as indicated in the MyST Supported Artifacts. At deploy-time if you have a partition defined in the MyST metadata for any given SCA Artifact (aka Composite) and the partition does not exist, it will be auto-created. If the composite.partition is defined in the Maven pom.xml at SCA build-time and the MyST plugin was used to push the artifact to Studio, MyST will automatically discover the partition and use that to verify it exists at deploy-time and create it if it does not. For example, to set Stock as the partition name for a given SCA artifact, the following can be added within the properties of a Maven pom.xml.
Alternatively, you could use the MyST Studio Control > Custom action under a given Platform Instance to create the partition directly before deployment with an action name of create-partition and setting the name as a property (e.g. -Dsca.composite.partition=Stock).