Azure Devops release pipeline - Get artifact variable

I've a Release pipeline in AzureDevops which pulls it's artifact from Azure Container Registry.

By creating a new release, at the "dockerbuild" needs to be selected which image from ACR shall be chosen:

Currently i can't access the variable "dockerbuild" with it's value "1358 in the release pipeline. How do i need to write this, for access from powerShell?