Hello there,
You can put the name of the application on the alternate shell:s: line in your .rdp file, e.g.
alternate shell:s:notepad.exe
This article discusses the connection settings that are stored in the Default.rdp file on Windows Server 2003-based and Windows XP-based computers. https://learn.microsoft.com/en-GB/troubleshoot/windows-server/remote/remote-desktop-protocol-settings
Supported RDP properties with Remote Desktop Services https://learn.microsoft.com/en-us/windows-server/remote/remote-desktop-services/clients/rdp-files
Similar discussion here https://social.technet.microsoft.com/Forums/windows/en-US/dc34c6b4-78ac-4928-bb23-3e4ca6868bc3/start-rdp-program-only?forum=w7itprogeneral
Hope this resolves your Query !!
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