assigning function pointers using designated initializers

How does one assign a function pointer using designated initializers? The following works with assignment but not with designated initializers.

#include <signal.h>void handler(int){}int main() {#if 1 // Setting to 1 does not work  struct sigaction st{.sa_handler=handler};#else  struct sigaction st;  st.sa_handler = handler;#endif}

<source>:7:23: error: expected primary-expression before '.' token    7 |   struct sigaction st{.sa_handler=handler};      |                       ^