How to define named parameter as [ref] in PowerShell

I'm trying to use [ref] named parameters. However, I am getting an error:

workflow Test
{
    Param([Parameter(Mandatory=$true)][String][ref]$someString)

    write-verbose $someString -Verbose
    $someString = "this is the new string"
}

cls
$someString = "hi"
Test -someString [ref]$someString
write-host $someString

#Error: Cannot process argument transformation on parameter 'someString'. Reference type is expected in argument.

How can I fix this problem?

I noticed that you are using a "workflow" in your example of a [ref] parameter. For simplicity, let's call it a "function" and get back to "workflow" later.

There are three things you need to change in your code:

1. When passing a [ref] parameter to function, you need to enclose the parameter in parenthesis ().
2. When using a [ref] parameter within a function refer to $variable.value
3. Remove [string] type from your parameter definition. It can be a [string], or [ref], but not both.

Here is code that works:

function Test
{
    Param([Parameter(Mandatory=$true)][ref]$someString)

    write-verbose $someString.value -Verbose
    $someString.value = "this is the new string"
}
cls
$someString = "hi"
Test -someString ([ref]$someString)
write-host $someString

As for "workflows". They are very restricted, read PowerShell Workflows: Restrictions. In particular you can't invoke a method on an object within workflow. This will break the line:

$someString.value = "this is the new string"

I don't think that using [ref] parameters in a workflow is practical, because of workflow restrictions.