Doubt with the library esp8266wifi and Station Mode
This is the function prototype:



Per C++ rules, in order to supply an argument for bssid, you have to pass an argument for channel as well, but connect is optional since it's positioned after your target argument. This should not be a problem though since you can just pass the default value of channel (i.e. 0) into the function; the channel argument is only relevant if you enter a value between 1 and 13 inclusive. 

So your call should basically look like:

char ssid[] = "ssid";
char pwd[] = "pwd";
uint8_t bssid[] = {0x00, 0x1b, 0x44, 0x11, 0x3a, 0xb7};
WiFi.begin(ssid, pwd, 0, bssid);