I am working on creating custom vsts build task using Hosted agent and powershell script. I just want to add reference of existing task available out of the box in vsts (Publish Build Artifacts). Is there any way to reference this task in our custom task? Or I just have to implement the functionality provided by PublishBuildArtifacts manually?
The way to reference another task us to grab the sources from the Task Repository on GitHub and package them with your own task (in a subfolder). You'll need to copy the inputs from their task.json and merge them into yours if you want to allow other users to configure the fields exactly the same way as the other task is doing.
You can find the task implementations here: https://github.com/Microsoft/vsts-tasks make sure you select the right branch, the master branch is the bleeding edge, and it may contain a version of the tasks that is not fully battle tested or may not be compatible with the latest version of the agent that has been released (or the minimal agent version you are targeting).
Or you can grab the implementation from a build agent's tasks directory.
Remember that for certain functionality, the VSTS Task SDK has built-in methods to upload artefact, which may make your life easier if you decide to implement the functionality on your own.
The team that built the agent has been pretty specific about making sure that tasks are self-contained and need to package their own dependencies or flag a demand. This is to ensure that each task can evolve and change independently.