query user in trusting domain

Hi I have a script to read through a CSV file and return the SamAccountName for users based on their DisplayName.

My computer is in one domain but I am trying to run the query against users in a trusting domain.  

Import-Module ActiveDirectory
Function Get-OSCSamAccountName
{
<#
 	.SYNOPSIS
        Get-OSCSamAccountName is an advanced function which can be used to get active directory user SamAccount name.
    .DESCRIPTION
        Get-OSCSamAccountName is an advanced function which can be used to get active directory user SamAccount name.
    .PARAMETER  CsvFilePath
		Specifies the path you want to import csv files.
    .EXAMPLE
        C:\PS> Get-OSCSamAccountName -CsvFilePath C:\Script\Users.csv

		This command will list all active directory user SamAccount Name info.
#>
    [CmdletBinding()]
    Param
    (
        [Parameter(Mandatory=$true)]
        [String]$CsvFilePath
    )

    If($CsvFilePath)
    {
        If(Test-Path -Path $CsvFilePath)
        {
            #import the csv file and store in a variable
            $Names = (Import-Csv -Path $CsvFilePath)."Full Name"
            
            Foreach($Name in $Names)
            {
                # $Name = $Name.Replace(" ","") -split ","
                $Name = $Name -split " "
                $FirstName = $Name[0].Trim()
                $LastName = $Name[1].Trim()
                $UserName = $FirstName + " " + $LastName
                #Retrieve the ad users based on previous two variables.
                $SamAccountName = Get-ADUser -Server mydomain.com -Filter{ Surname -eq $LastName -and GivenName -eq $FirstName} |`
                Select -ExpandProperty SamAccountName 
                                
                If($SamAccountName -eq $null)
                {
                    $SamAccountName = "NotFound"
                }

                #Output the result
                New-Object -TypeName PSObject -Property @{DisplayName = $UserName
                                                          SamAccountName = $SamAccountName
                                                         }
                    
            }
        }
        Else
        {
            Write-Warning "Cannot find path '$CsvFilePath' because it does not exist."
        }
    }
}

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When I run the above script with without the "-Server" parameter for Get-ADUser It works fine but only searches the domain my PC is in.

$SamAccountName = Get-ADUser -Filter{ Surname -eq $LastName -and GivenName -eq $FirstName} |`
                Select -ExpandProperty SamAccountName 

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I read on this site that you can add the "-Server" parameter and the DNS name of the Domain you want to search:

$SamAccountName = Get-ADUser -Server mydomain.com -Filter{ Surname -eq $LastName -and GivenName -eq $FirstName} |`
                Select -ExpandProperty SamAccountName 

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 However when I add the "-Server" parameter I get the following error:

Get-ADUser : The input object cannot be bound to any parameters for the command either because the command does not take pipeline input or the input and its properties
do not match any of the parameters that take pipeline input.