Transfer serial data to struct variable in Arduino
You read the serial port one char at time, and assign it to test1.pt[i], where i is an index varying from 0 to 15.

And test1 cannot be const, because you are going to modify it with the serial data.

Here is a test code showing how to do it.

struct testing_var 
{
    const char *name;
    byte key[32];
    byte pt[16];
};

//  test1 cannot be 'const'
static testing_var test1 = {
    .name        = "Speck-128-ECB",
    .key         = {0x0f, 0x0e, 0x0d, 0x0c, 0x0b, 0x0a, 0x09, 0x08,
                    0x07, 0x06, 0x05, 0x04, 0x03, 0x02, 0x01, 0x00},
    .pt   = {0x6c, 0x61, 0x76, 0x69, 0x75, 0x71, 0x65, 0x20,
                    0x74, 0x69, 0x20, 0x65, 0x64, 0x61, 0x6d, 0x20},
};

void setup() {
  Serial.begin(9600);
  while(!Serial);
}

void loop() {
  Serial.println("Enter 16 chars:");
  //  Read 16 chars from Serial
  for (int i=0; i &lt; 16;) {
    if (Serial.available()) {
      test1.pt[i++] = Serial.read();
    }
  }
  // Do something with the data, like print it.
  for (int i = 0; i &lt; 16; i++) {
    Serial.print(char((test1.pt[i])));
  }
  Serial.println();
}