This answer offers two alternatives to using Prem's answer. Although, I feel the proper application of Prem's answer would be the easiest to implement.
Alternative #1
You can use the following (or edit an existing) AutoUnattend.xml file to have the on-screen keyboard open automatically. This file needs to be placed in the root directory of the volume on the flash drive installer. Once the on-screen keyboard appears, enter the shift+F10 key combination to open a Command Prompt window. An example AutoUnattend.xml file is given below and is also available for download from Pastebin.
<?xml version="1.0" encoding="utf-8"?>
<unattend xmlns="urn:schemas-microsoft-com:unattend">
<settings pass="windowsPE">
<component name="Microsoft-Windows-Setup" processorArchitecture="amd64" publicKeyToken="31bf3856ad364e35" language="neutral" versionScope="nonSxS" xmlns:wcm="http://schemas.microsoft.com/WMIConfig/2002/State" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<UserData>
<ProductKey>
<WillShowUI>Always</WillShowUI>
</ProductKey>
</UserData>
<RunAsynchronous>
<RunAsynchronousCommand wcm:action="add">
<Order>1</Order>
<Path>cmd /c "for %i in (x g f e d c) do if exist %i:\windows\system32\osk.exe (start /b %i:\windows\system32\osk.exe & exit)"</Path>
</RunAsynchronousCommand>
</RunAsynchronous>
</component>
</settings>
</unattend>
For this to work, the files osk.exe and OskSupport.dll files need to exist in the Windows\System32 folder on drive X:, G:, F:, E:, D: or C:. The drives are searched in this order. This was tested using the files from either Windows 10 22H2 or Windows 11 22H2. I would recommend creating a Windows\System32 folder in the volume on the flash drive installer, then copying these two files to this folder.
Since an AutoUnattend.xml file can prevent shutdown down through the GUI, one may consider also copying the file shutdown.exe to the flash drive. This would allow the command below to be used to shutdown the computer. Here, the drive letter assigned to the USB Windows installer flash drive is the letter D:. If your drive letter is different, then make the appropriate substitution.
d:\windows\system32\shutdown /s /t 0
Alternative #2
You can edit the registry to have the on-screen keyboard open automatically when a Command Prompt windows opens. For this to work, the files osk.exe and OskSupport.dll files need to exist in the X:Windows\System32 folder. This was tested using the files from either Windows 10 22H2 or Windows 11 22H2.
To make the changes, open a Command Prompt windows as an Administrator, then enter the following commands. The commands copy the two files and edit the registry. (Here, the shutdown.exe file is also copied. See Alternative #1 for an explanation.)
Note: If the drive letter assign to the Windows installer USB flash drive is not D:, then make the appropriate substitution when entering d:\sources\boot.wim.
cd /d %userprofile%
md offline
dism /mount-wim /wimfile:d:\sources\boot.wim /index:2 /mountdir:offline
copy c:\windows\system32\osk.exe offline\windows\system32
copy c:\windows\system32\osksupport.dll offline\windows\system32
copy c:\windows\system32\shutdown.exe offline\windows\system32
reg load HKLM\OFFLINE offline\Windows\System32\Config\SOFTWARE
reg add "HKLM\OFFLINE\Microsoft\Command Processor" /v AutoRun /t REG_EXPAND_SZ /d osk /f
reg unload HKLM\OFFLINE
dism /unmount-image /mountdir:offline /commit
rd offline
If wish to shutdown the computer after booting from the USB flash drive Windows installer, then you can enter the command below.
shutdown /s /t 0