Add a ScriptMethod Helper to query PSCustomObject - powershell

Question

I am trying to add some simple helper methods to an existing PSCustomObject.
My goal is to replace something like this:
$Persons.Where{$_.FirstName -like "*Test*"}
With this:
$Persons.GetFirstName("Test")
I am having a lot of trouble figuring out a working ScriptMethod to use with add-member.
Here is my example code, along with all of the variations I have tried so far:
$Persons = [PSCustomObject]#()
$Person1 = [PSCustomObject]#{
FirstName = 'John';
LastName = 'Doe';
Age = 27
}
$Person2 = [PSCustomObject]#{
FirstName = 'Test';
LastName = 'Dummy';
Age = 27
}
$Persons += $Person1
$Persons += $Person2
$Persons
#FirstName LastName Age
#--------- -------- ---
#John Doe 27
#Test Dummy 27
#Get using a regular where method to confirm
$Persons.Where{$_.FirstName -like "*Test*"}
#FirstName LastName Age
#--------- -------- ---
#Test Dummy 27
#Add the Helper Method GetFirstName
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where{$_.FirstName -like "$Args[0]"}}
#Check if the Test record is returned
$Persons.GetFullName("Test")
#Use this to remove the method for the next try
$Persons.PSObject.properties.remove('GetFirstName')
#Other Permutations I have tried
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where($this.FirstName -like "$($Args[0])")}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({FirstName -like $Args[0]})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({FirstName -like $args(0)})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({FirstName -like "$Args[0]"})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({FirstName -like "$($Args[0])"})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({$this.FirstName -like $Args[0]})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({$this.FirstName -like "*$($Args[0])*"})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({$this.FirstName -eq $Args[0]})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({$_.FirstName -like $Args[0]})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where({$_.FirstName -like "$($Args[0])"})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where("FirstName -like ""$($Args[0])""")}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.where("$($_.FirstName) -like ""$($Args[0])""")}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.GetEnumerator().where{FirstName -like $Args[0]}}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.GetEnumerator().where{$this.FirstName -like $Args[0]}}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this.GetEnumerator().where{$_.FirstName -like $Args[0]}}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this | ?{$_.FirstName -like $Args[0]}}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this | ?({$_.FirstName -like $Args[0]})}
add-member -inputobject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {$this | ?({$_.FirstName -like "$($Args[0])"})}

Answer 1

There's one $Args per scope, so $Args inside .Where{ <in here> } does not actually contain the input arguments to the script method - declare a named parameter instead and it'll work:
Add-Member -InputObject $Persons -MemberType ScriptMethod -Name GetFirstName -Value {
param([string]$fnArg)
$this.Where{$_.FirstName -like $fnArg}
} -Force