This will modify the C: disk volume, halving it to create space for another partition D:, and enable BitLocker

Add-ExtraDiskPartition.ps1

<#
.Synopsis
   Divides the Partition hosting C: into two partitions

.DESCRIPTION
   This script;
    - moves the optical drive to Z:
    - divides c: into two partitions
    - creates partition & formats D: with the label 'Local Data'
    - enables BitLocker for D: (assumes that the recovery key is stored in AD)

.EXAMPLE
   There are no parameters:

   .\Add-ExtraDiskPartition.ps1

.NOTES
   Author: Stu Mace <stum@datacom.co.nz>
#>

# change CD to Z:, DriveType=5 indicates an optical drive, it is assumed there is at most only one
# this is required because if present the optical drive will typically occupy D:
try
{
    Get-CimInstance -Class Win32_Volume -Filter 'DriveType=5' |
        Select-Object -First 1 |
        Set-CimInstance -Arguments @{DriveLetter='Z:'}
}
catch
{
    Write-Output 'No optical drive found'
}

# get the disk number of the C: drive, required later
$CDiskNumber = (Get-Partition -DriveLetter C).DiskNumber
# get the total capacity of the C: drive
$maxDiskSize = (Get-Disk | Where Number -eq $CDiskNumber).Size

# check that the drive is health before doing any partitioning
if ((Get-Volume -DriveLetter C).HealthStatus -eq 'Healthy')
{
    # resize the partition by half
    $partitionNumber = (Get-Partition -DriveLetter C).PartitionNumber
    $maxPartitionSize = (Get-PartitionSupportedSize -DriveLetter C).SizeMax

    # if the current partition is the same size as disk, a resize is required
    # a margin of error of 10GB is used to account for system reserved partitions
    if (($maxDiskSize - $maxPartitionSize) -lt (10 * 1024 * 1024 * 1024))
    {
        # resize the C partition (make half the size)
        Resize-Partition -DriveLetter C -Size ($maxPartitionSize / 2 )

        # create & format the new partition without prompting, based on:
        # https://stackoverflow.com/questions/42620986/formatting-a-disk-using-powershell-without-prompting-for-confirmation
        $partition = New-Partition -DiskNumber $CDiskNumber -UseMaximumSize
        $partition | Format-Volume -NewFileSystemLabel 'Local Data' -Confirm:$false
        $partition | Set-Partition -NewDriveLetter D
    }
    
    # enable bitlocker if protection is not on
    if ($null -eq (manage-bde.exe -status d: | select-string 'Protection On'))
    {
        manage-bde -on D: -recoverypassword -skiphardwaretest
    }
}
else
{
    Write-Warning ("C: drive status reports not healthy, status: {0}" -f (Get-Volume -DriveLetter C).HealthStatus)
}