How can I concatenate these two values as a string?
You can use the c function sprintf(, , ...)
The char array has to be big enough to hold your text plus a null to terminate the string.

#include &lt;Arduino.h&gt;

void setup() {
  Serial.begin(115200);
  delay(1000);
  char out[20];
  sprintf(out, "Device-%08X",ESP.getChipId()); // %08X\n formats the hex and pads with 0s.

  Serial.println(out);
}

void loop() {}


Output:

Device-005D4D9D