Use Variable for Select-Object or Format-Table

Note:

• The answer applies analogously to the Select-Object cmdlet, which is what should be used to extract properties for later programmatic processing.
  Format-* cmdlets, such as Format-Table in this case (whose built-in alias is ft), are only intended to produce for-display formatting; see this answer for more information.

Theo has provided the solution in a comment:

$x = 'Id', 'ProcessName', @{ Name = 'CPUx' ; Expression = { $_.CPU } }
Get-Process | Format-Table -AutoSize $x  # same as: ft -a $x

That is, the answer to the linked question applies in your case as well, even though it happens to use only literal strings as property names:

Directly construct an array of property names and calculated properties, using ,, the array constructor operator, which allows you to use any mix of literals and variables; e.g.:

$pName = 'processName'                               # property name
$pCpu = @{ Name = 'CPUx' ; Expression = { $_.CPU } } # calculated property

$x = 'Id', $pName, $pCpu

Do not start with a single string that you split into an array of tokens with .Split(), because it will limit you to property names, given that the tokens are invariably strings.

As an aside:

The array binds positionally to Format-Table's (ft's) -Property parameter, and you can easily discover that as well as the fact that an array of property names / calculated properties is discovered as follows:

PS> Format-Table -?
...

SYNTAX
    Format-Table [[-Property] <System.Object[]>] ...

DESCRIPTION
...

• The outer [...] tells you that the parameter is optional as a whole.

• The [...] around -Property tells you that specifying the parameter name explicitly is optional, i.e. that positional binding is supported.

• <System.Object[]> tells you that an array ([]) of System.Object instances is expected as the type of the argument(s).

To get more information about what objects, specifically, may be passed, inspect the parameter individually:

PS> Get-Help Format-Table -Parameter Property

-Property <System.Object[]>
    Specifies the object properties that appear in the display and the order 
    in which they appear. Type one or more property names, separated 
    by commas, or use a hash table to display a calculated property. 
    Wildcards are permitted.
...