IPC is the only way as David mentioned. As far as IPC options it depends upon whether the apps are running on the same machine or across the network, internally or externally. Refer to the docs he mentioned for the options available and determine the one(s) to use based upon your needs. For local machines you'll probably use named pipes, for remote machines then perhaps something simple like a TCP server. Across the INet then you're looking at REST. WCF would be an option as well but be aware that WCF Server is not supported in .NET Core/5+ so I would not recommend going that route.
How do two WinForms applications communicate with each other?
I wrote two WinForms programs and installed them on two computers.How can one WinForms application get the other WinForms application to output some string on the interface, such as a string "hello"?