ArgTransformTest.ps1

function Main
{
    #1.No attribute
    $a = 5;
    Write-Host ($a) #5

    #2.With Add Value attribute
    [AddValue(5)]
    $b = 5;
    Write-Host $b #5+5 =10

    #3.Assign variable after declaration
    [AddValue(5)]
    $c = 1 + 1;
    $c = 10 + 5
    Write-Host $c #15+5 = 20
}

Add-Type -TypeDefinition @'
    using System;
    using System.Management.Automation;

    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Property)]
    public class AddValueAttribute : ArgumentTransformationAttribute
    {
        private int addValue;
        public AddValueAttribute(int addValue)
        {
            this.addValue = addValue;
        }
        public override object Transform(EngineIntrinsics engineIntrinsics, object inputData)
        {
            return (int) inputData + addValue;
        }
    }
'@

Main