how to instert "x-forwarded-for" date to http header in physical l4 switch?

Question

In the L4 switch, there is an 'x-forwarded-for' function that puts the client source ip address in the http header.

The l4 switch can only know layer 4 information, so I'm curious how to put the x-forwarded-for information http header(http header is in the layer 7!! ). Even in https, the http header is encrypted, how can L4 switch decrypt this encrypted http header, insert x-forwared-for information, and send a packet to the backend?

Thanks!