Question: I got a .xll file that I can easily add to excel by doing this: Options > Addins > Browse > double click .xll file It gets imported + activated (and it remains in my excel addins every time I close and open Excel). This is the manual way I try to replace with a script. PowerShell $excel=New-Object -ComObject excel.application $excel.RegisterXLL("C:\temp\v-0.0.1-20210906\LS-ZmqRtd-AddIn64.xll") $excel.Visible = "$True" #$excel.Quit()  This will create an instance of Excel, register the XLL (I get a "true" in my console) and show the created instance. But when I then go to AddIns, the AddIn isn't there. Python xl = win32com.client.gencache.EnsureDispatch("Excel.Application") xl.Visible = True xl.RegisterXLL( "C:/Users/michael.k/Desktop/v-0.0.1-20210906/LS-ZmqRtd-AddIn64.xll" ) wb = xl.Workbooks.Open("C:/Users/michael.k/Desktop/v-0.0.1-20210906/Test.xlsx")  But this behaves like the Powershell script. So.. how can I add my .xll file into Excel to stay there permanently? Any suggestions? Thanks in advance!

Answer: Thanks to the link Charles Williams gave me, I was able to do it a little bit different. You can easily create a registry key to let excel know that it should run the .xll-file. # initializing new variables $req_path = Get-Item -Path Registry::HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Excel\Options | Select-Object -ExpandProperty Property $newest_version = (Get-ChildItem -Path I:\Software\LS-ZmqRtd\Test\ -Directory | sort lastwritetime | Select -Last 1).Name $full_path_new_version = '/R "I:\Software\LS-ZmqRtd\Test\'+$newest_version+'\LS-ZmqRtd-AddIn64.xll"' $only_opens = @() [bool]$ls_zmqrtd_found = $false [bool]$ls_zmqrtd_updated = $false $count_opens = 0 # welcome message echo ">> checking if the LS-ZmqRtd addin is installed and has the newest version.." Start-Sleep -s 5 # check if there are regkeys that contain 'OPEN' in their name (if yes, add them to the $only_opens array) foreach ($entry in $req_path) { if ($entry -like "OPEN*") { $only_opens += $entry } } if (!$only_opens) # check if array is empty (if yes, add the new regkey for LS-ZmqRtd) { echo ">> the LS-ZmqRtd addin couldn't be found.. adding it to excel now." Start-Sleep -s 2 New-ItemProperty -Path HKCU:\Software\Microsoft\Office\16.0\Excel\Options -Name OPEN -PropertyType String -Value $full_path_new_version echo ">> addin was added to excel successfully - this requires Excel to be fully closed and re-opened." } else # if no, check if one of the regkeys have the LS-ZmqRtd path value (if found, set $ls_zmqrtd_found to true - else remain false) { foreach ($open in $only_opens) { $value = (Get-ItemProperty -Path "HKCU:\Software\Microsoft\Office\16.0\Excel\Options" -Name $open).$open if ($value -eq $full_path_new_version) { $ls_zmqrtd_found = $true } else { echo ">> found an old version of LS-ZmqRtd.. replacing it with the new one now." Start-Sleep -s 2 Set-ItemProperty -Path HKCU:\Software\Microsoft\Office\16.0\Excel\Options -Name $open -Value $full_path_new_version $ls_zmqrtd_updated = $true } $count_opens += 1 } if ($ls_zmqrtd_found -eq $true) # if $ls_zmqrtd_found is true, there is nothing to do { echo ">> found that the newest version of LS-ZmqRtd is already installed - nothing to do here." } elseif ($ls_zmqrtd_updated -eq $true) { echo ">> updated LS-ZmqRtd to the newest version - an update requires Excel to be fully closed and re-opened." } else # if $ls_zmqrtd_found is false, increment the last OPEN's number by 1 and add the new reqkey for LS-ZmqRtd { $new_reg_key = "OPEN" + ($count_opens+1) echo ">> the LS-ZmqRtd addin couldn't be found.. adding it to excel now." Start-Sleep -s 2 New-ItemProperty -Path HKCU:\Software\Microsoft\Office\16.0\Excel\Options -Name $new_reg_key -PropertyType String -Value $full_path_new_version echo ">> addin was added to excel successfully - this requires Excel to be fully closed and re-opened." } }  This script checks if the .xll-File is already named in a registry key.  If yes and it has our newest provided version -> do nothing If yes but the version is old -> update the registrey key's value If no -> create the registry key and set the value to our newest provided version