Wifi REST API GET call - how to concatenate and pass URL parameters?
Concatenating the string data in memory before sending it shouldn't be necessary. Instead, you should just be able to use client.print() (or equivalent) to output each part of the request in sequence. For example:

client.print("GET ");
client.print(page);
client.print(" HTTP/1.1\r\n");

client.print("Host: ");
client.print(serverName);
client.print("\r\n");

// etc.


That should let you output any combination of HTTP headers you need.