How to Send RPC By System call

I want to send a rpc call by using system call like:
or something else, just not directly use RPC function
UFUNCTION(reliable, server, WithValidation)
void ServerStartFire();

because, I am not use full actor object to call rpc, I use
UMinimalClient* m_UEMinClient;
to connection the server, join the session, and call the rpc function,
i use this m_UEMinClient as a robot, because i want to do some pressure testing by generate many robot and call specify RPC functions through robot clients