function getAvailableAllocation(address _addr)
  constant
  returns (uint ethAvailableAllocation)
{
  return investorActions.getAvailableAllocation(_addr);
}

onSubmit = async event => {
  event.preventDefault();

  const accounts = await web3.eth.getAccounts();

  await fund.methods.getAvailableAllocation(this.state.value).call({
    from: accounts[0],
    address: this.state.value
  }).then(console.log);

<form onSubmit={this.onSubmit}>
  <h4>Available allocation</h4>
  <h5>Get the remaining available amount in Ether that an investor can subscribe for</h5>
  <div>
    <label>Your ethereum address</label>
    <input
      value={this.state.address}
      onChange={event => this.setState({ address: event.target.value })}
    />
    </div>
    <button>Get available allocation</button>
</form>