`PSConsoleReadLine` has a `GetSelectionState` method but no `SetSelectionState` method. `Set-PSConsoleReadLineSelectionState` will enable you to set the selection state and also specify if the selection was done backwards or forwards (by default it is forwards).

Set-PSConsoleReadLineSelectionState.ps1

Function Set-PSConsoleReadLineSelectionState
{
    param(
        [Parameter(Mandatory)][int]$Start, 
        [Parameter(Mandatory)][int]$Length,
        [ValidateSet('Backward', 'Forward')][string]$Direction='Forward'
    )

    $startCursorPosition,$selectChar = switch ($Direction)
    {
        'Backward' {
            $Start + $Length; [Microsoft.PowerShell.PSConsoleReadLine]::SelectBackwardChar
        }
        'Forward' {
            $Start; [Microsoft.PowerShell.PSConsoleReadLine]::SelectForwardChar
        }
    };

    [Microsoft.PowerShell.PSConsoleReadLine]::SetCursorPosition($startCursorPosition);
    [Microsoft.PowerShell.PSConsoleReadLine]::SetMark($null, $null);

    (1..$Length) | ForEach-Object {
        $selectChar.Invoke($null, $null);
    }
}

<#

To select the first five characters do:

Set-PSConsoleReadLineSelectionState 0 5 Forward

#>