Hi,
XML file is not directly assignable as a file via OMA-URI. The only supported CSP is via the Settings Catalog, ADMX-backed policies, or SCCM. But best is to use Settings catalog.
Here is example for XML - https://learn.microsoft.com/en-us/windows/configuration/taskbar/pinned-apps?tabs=intune&pivots=windows-11#taskbar-layout
And here are examples for Win11- https://learn.microsoft.com/en-us/windows/configuration/taskbar/pinned-apps?tabs=intune&pivots=windows-11#example-add-pins [you could scroll down there are a lot of example, just get the XML edit for your need and then create a settings catalog.
Here is guidance which settings should be used - https://learn.microsoft.com/en-us/windows/configuration/taskbar/pinned-apps?tabs=intune&pivots=windows-11#deploy-the-taskbar-configuration
Before deploying to pilot or production you could also test XML locally using powershell.
Import-StartLayout -LayoutPath "C:\Path\To\Your\LayoutModification.xml" -MountPath $env:SystemDrive\