Split a CSV file into multiple files and and give a unique name based on a Appname field. Fetch only account column in the newly created Appname-file

I have a query in PowerShell, I need to split a single CSV file into different files based on the appname and with only account information and ignore the rest of the columns. Please help me here to solve it from a PowerShell script.

$InputFilePath = "E:\Test_csv\test.csv"
$SplitByColumnName = "AppName" #Enter ColumnName here on basis of which you want to split.

$data = Import-Csv $InputFilePath | Select -ExpandProperty $SplitByColumnName -Unique

$a = $data | select 

ForEach ($i in $a)
{  
  $FinalFileNamePath = "E:\Test_csv\" + $i + ".CSV" #This is where you would keep the splitted files.

  Import-Csv $InputFilePath | where {$_.$SplitByColumnName -eq $i } | Export-Csv $FinalFileNamePath -NoTypeInformation  
}

I tried the above code but how to get only the account name instead of all the columns?

$Csv = @'
name,  account,  Appname, count, users, List
name1, account1,     123,     1,     2, abc
name1, account2,     123,     2,     3, z
name1, account3,     123,     3,     6, xc
name1, account4,     123,     5,     7, df
name1, account5,     123,     5,     8, rg
name1, account6,     123,     7,     0, sfg
name2, account1,     456,     1,     1, dfg
name3, account1,     789,     3,     7, rt
name3, account2,     789,     7,     1, ert
'@

For what you tried:
You will need to filter (Where-Object) upon the Appname and select (Select-Object) upon the account name. If you use appropriate variable names (rather than $a and $i), things might get clearer:
^{(Personally, I value variable names more than comments)}

$SplitByColumnName = "AppName" # Enter ColumnName here on basis of which you want to split.

$data = ConvertFrom-Csv $Csv # = Import-Csv $InputFilePath
$AppNames = $Data | Select-Object -ExpandProperty $SplitByColumnName -Unique

ForEach ($Appname in $Appnames) {
  $FinalFileNamePath = "E:\Test_csv\" + $Appname + ".CSV" #This is where you would keep the splitted files.
  Import-Csv $InputFilePath |
    Where-Object Appname -eq $Appname |
    Select-Object Account |
    Export-Csv $FinalFileNamePath -NoTypeInformation  
}

Avoid wrapping cmdlet pipelines
Note that wrapping cmdlets (as Import-Csv and Export-Csv) in a loop is expensive as it (re)opens and closes the concerns files each cycle.
To avoid this in your case, you might use the steppable pipeline and create multiple output streams to fill the concerned exported csv files.

$Pipeline = @{}
ConvertFrom-Csv $Csv | # Import-Csv $InputFilePath |
ForEach-Object -Process {
    if (!$Pipeline.Contains($_.Appname)) {
        $Pipeline[$_.Appname] = {
            Select-Object Account | Export-CSV -NoType -Path "E:\Test_csv$($_.Appname).CSV"  
        }.GetSteppablePipeline()
        $Pipeline[$_.Appname].Begin($True)
    }
    $Pipeline[$_.Appname].Process($_)
} -End {
    foreach ($Key in $Pipeline.Keys) { $Pipeline[$Key].End() }
}

For background and details, see: Mastering the (steppable) pipeline