How to include better bit mixer for unordered map C++14

How to incorporate the following bit mixing code in order to minimize the hash collisions inside the unordered_map?

UInt64 MurmurHash3Mixer( UInt64 key )
  {
  key ^= (key >> 33);
  key *= 0xff51afd7ed558ccd;
  key ^= (key >> 33);
  key *= 0xc4ceb9fe1a85ec53;
  key ^= (key >> 33);

  return key;
  }

Posted code was referred from the site.

Following is the full code (testing code) just to get 'MurmurHash3Mixer' included into the 'std::unordered_map' hashing scheme.

#include <iostream>
#include <unordered_map>

uint64_t MurmurHash3Mixer( uint64_t key ) { 
    key ^= (key >> 33);
    key *= 0xff51afd7ed558ccd;
    key ^= (key >> 33);
    key *= 0xc4ceb9fe1a85ec53;
    key ^= (key >> 33);

    return key;
}


int main() {

    uint64_t tx = 10L;
    std::unordered_map<uint64_t, uint64_t> ht; 
    ht.insert(std::make_pair<uint64_t, uint64_t>(tx, 20));
    std::cout << ht[tx] << std::endl;

    return 0;
}