How to iterate over Object properties of an input parameter

I have an Azure Automation Powershell Workflow:

workflow wf
{
  param(
    [parameter(Mandatory=$True)]
    [object] $p
  )
  inlinescript
  {
    # ...
  }
}

I am testing it using the Test pane and passing the value as {"FirstName": "John", "LastName": "Smith"}. I have also tried {"FirstName"="John";"LastName"="Smith"} and adding the @ in front of both cases.

According to this

If your runbook has an object type input parameter, then use a PowerShell hashtable with (name,value) pairs to pass in a value. For example, if you have the following parameter in a runbook: [Parameter (Mandatory = $true)][object] $FullName then you pass the following value to the parameter: @{"FirstName"="Joe";"MiddleName"="Bob";"LastName"="Smith"}

But in all of my tests $p is null.

How do I define an actual object, pass it in, and then iterate over the property names and values?

To access the input parameters within an activity of the PowerShell Workflow, you have to do via the $Using keyword. Once I figure that one out I was able to see the type was being passed in as a string. So I had to convert from a json string to an object by using ConvertFrom-Json cmdlet. I was then able to iterate through the properties by getting the Properties property of the PSObject:

$pObj = $Using:p | ConvertFrom-Json
ForEach ($pr in $pObj.PSObject.Properties)
{
   #$pr.Name
   #$pr.Value
}