void transfer_token(account_name receiver, asset quantity, std::string memo) {

    action(
            permission_level{_self, N(active)},
            N(eosio.token),
            N(transfer),
            std::make_tuple(_self, receiver, quantity, memo)
    ).send();
}

cleos set account permission <contract> active '{"threshold": 1,"keys": [{"key": <public_key>,"weight": 1}],"accounts": [{"permission":{"actor": <user>,"permission":"active"},"weight":1}]}' owner -p <contract>