JSON Array in PowerShell

The outer [] is a list in JSON and you're trying to use @{}, which is a hashtable in PowerShell. Use @() which is an array (list) in PowerShell:

$Body = @(
    @{
        FirstName='Test'
        LastName='Account2'
    }
)
ConvertTo-Json -InputObject $Body

(and I use -InputObject instead of piping, because PowerShell is obsessed with flattering lists, and drops the list otherwise).

I had some problems adding new items to the Powershell list. I tried to add an item using the .add() function. This returned the error "Collection was of a fixed size."

Adding items to the list is just simple by using the += symbol.

$Body += @{FirstName='Test2'; LastName='Account3'}

That simple.