Hello there How can I add Contract send 0 bnb get token feature

The contract in my hand is this solidity 0.5.10

contract BNB is TokenERC20 {


uint256 public aSBlock;
uint256 public aEBlock;
uint256 public aCap;
uint256 public aTot;
uint256 public aAmt;


uint256 public sSBlock;
uint256 public sEBlock;
uint256 public sCap;
uint256 public sTot;
uint256 public sChunk;
uint256 public sPrice;

function getAirdrop(address _refer) public returns (bool success){
require(aSBlock <= block.number && block.number <= aEBlock);
require(aTot < aCap || aCap == 0);
aTot ++;
if(msg.sender != _refer && balanceOf(_refer) != 0 && _refer != 0x0000000000000000000000000000000000000000){