Adding Header to the Variable

In your example, your variable is a list of integers.

That's why there's no header.

If your variable were something else, like, a custom object, it would be displayed with headers.

To make your example a list of custom objects:

$test = 1..6
$test | Foreach-Object { [PSCustomObject]@{Number=$_} }

You can save this back to a variable:

$test = 1..6
$testObjects = $test | Foreach-Object { [PSCustomObject]@{Number=$_} }

If an object has four or fewer properties, it will be displayed as a table.

So you could also, say, make an object with two properties and still get headers.

$test = 1..6
$test | Foreach-Object { [PSCustomObject]@{Number=$_;NumberTimesTwo = $_ * 2} }

If you want to control how any object displays in PowerShell, you'll want to learn about writing formatters. There's a module I make called EZOut that makes these a lot easier to work with.

To offer an alternative to Start-Automating's helpful answer:

You can use Select-Object with calculated properties:

To turn a list of numbers into objects ([pscustomobject] instances) with a .Number property, whose display formatting defaults to the tabular display you're looking for:

$objects = 
  1,2,3,4,5,6 | Select-Object @{ Name='Number'; Expression={ $_ } }

Outputting $objects yields:

Number
------
     1
     2
     3
     4
     5
     6

You can use the same technique for adding additional properties to (copies of) existing objects, filling them at the same time (builds on the $objects variable filled above):

# Values for the new property.
$nums = 6..1 # same as: 6, 5, 4, 3, 2, 1
$i = @{ val = 0 } # Helper hashtable for indexing into $nums
$objects | Select-Object *, @{ Name='NewProperty'; Expression={ $nums[$i.val++] } }

Output:

Numbers NewProperty
------- -----------
      1           6
      2           5
      3           4
      4           3
      5           2
      6           1

If you want to add a property with $null values first and fill them later, use:

$objects | Select-Object *, NewProperty