How to draw a bitmap on a tft display?
As the function prototype you have show suggests, the data needs to be in a uint8_t array format.

void Adafruit_GFX::drawXBitmap(int16_t x, int16_t y,
            const uint8_t *bitmap, int16_t w, int16_t h,uint16_t color)


So your array data would look like:

const uint8_t myBitmap[] = { 0xff, 0xff, 0xc0, 0x00, 0xc0, 0xff,
    0xc0, 0xc0, 0xff, 0xff, 0xde, 0xad, 0xbe, 0xef,
    .... etc ....
    0xff, 0xff, 0xff, 0xff, 0xff
};


You then call the function with the needed parameters:

tft.drawXBitmap(10, 20, myBitmap, 100, 80, 0xFFFF);


That is assuming you want to place it at (top left corner) 10,20 and the bitmap is 100x80 pixels in size.  It will draw it in white (0xFFFF).