semantic ui react default selected options in dropdown

You were not far from result.

You can provide an array of values in the defaultValue props as the docs said.

defaultValue {number|string|arrayOf} Initial value or value array if multiple.

Here an example:

class YourComponent extends Component {
  componentWillMount() {
    this.setState({
      options: [
        {value:'1', text:'A'},
        {value:'2', text:'B'},
        {value:'3', text:'C'},
      ],
      selected: ['1', '2'], // <== Here, the values of selected options
    });
  }

  ...

  render() {
    return (
      <Form onSubmit={this.handleFormSubmit}>
        <Form.Dropdown
          placeholder="Select Options"
          fluid multiple selection
          options={this.state.options}
          onChange={this.handleMultiChange}
          defaultValue={this.state.selected} // <== here the default values
        />
        <Button type="submit">Submit</Button>
      </Form>
    );
  }
}

EDIT : Here is a live example

Works for single selections as well:

import React, { Component } from 'react';
import { render } from 'react-dom';
import { Form, Button } from 'semantic-ui-react';
import './style.css';

class App extends Component {
  componentWillMount() {
    this.setState({
      options: [
        {value:'1', text:'Lamborghini Aventador 2016'},
        {value:'2', text:'VW Beetle 1971'},
        {value:'3', text:'Ford Mustang'},
      ],
      selected: '1',
    });
  }

  render() {
    return (
      <div>
        <Form onSubmit={this.handleFormSubmit}>
          <Form.Dropdown
            placeholder="Select Options"
            defaultValue={this.state.selected}
            fluid selection
            options={this.state.options}
          />
          <Button type="submit">Submit</Button>
        </Form>
      </div>
    );
  }
}

render(<App />, document.getElementById('root'));