HFM minimum deposit is approximately 100 ZAR which is roughly 5 USD. But note that the account types you can open with 100 ZAR usually have high spreads, higher than the industry averages.
For instance, with 100 ZAR you can open the Premium or Cent Account where EUR/USD average spread is around 1.6 pips which is higher than the industry accepted average of 1.1 pips.
For lower spreads HFM has Pro Account with minimum deposit of around 2,000 ZAR where spreads on popular pairs like EUR/USD clock in lower at an average of 1.0 pip.
The lowest spread will be gotten on the PRO PLUS Account which has a minimum deposit around 5,000 ZAR and EUR/USD benchmark average spread is around 0.6 pips.