I hope this is still relevant, but here is my way.
For the custom dropdown, I created a new component and used method for "adding new option to the toolbar" from the documentation https://jpuri.github.io/react-draft-wysiwyg/#/docs
import React, { Component } from 'react';
import PropTypes from 'prop-types';
import { EditorState, Modifier } from 'draft-js';
class Placeholders extends Component {
  static propTypes = {
    onChange: PropTypes.func,
    editorState: PropTypes.object,
  }
  state = {
    open: false
  }
  openPlaceholderDropdown = () => this.setState({open: !this.state.open})
  addPlaceholder = (placeholder) => {
    const { editorState, onChange } = this.props;
    const contentState = Modifier.replaceText(
    editorState.getCurrentContent(),
    editorState.getSelection(),
    placeholder,
    editorState.getCurrentInlineStyle(),
    );
    onChange(EditorState.push(editorState, contentState, 'insert-characters'));
  }
  placeholderOptions = [
    {key: "firstName", value: "{{firstName}}", text: "First Name"},
    {key: "lastName", value: "{{lastName}}", text: "Last name"},
    {key: "company", value: "{{company}}", text: "Company"},
    {key: "address", value: "{{address}}", text: "Address"},
    {key: "zip", value: "{{zip}}", text: "Zip"},
    {key: "city", value: "{{city}}", text: "City"}
  ]
  listItem = this.placeholderOptions.map(item => (
    <li 
      onClick={this.addPlaceholder.bind(this, item.value)} 
      key={item.key}
      className="rdw-dropdownoption-default placeholder-li"
    >{item.text}</li>
  ))
  render() {
    return (
      <div onClick={this.openPlaceholderDropdown} className="rdw-block-wrapper" aria-label="rdw-block-control">
        <div className="rdw-dropdown-wrapper rdw-block-dropdown" aria-label="rdw-dropdown">
          <div className="rdw-dropdown-selectedtext" title="Placeholders">
            <span>Placeholder</span> 
            <div className={`rdw-dropdown-caretto${this.state.open? "close": "open"}`}></div>
          </div>
          <ul className={`rdw-dropdown-optionwrapper ${this.state.open? "": "placeholder-ul"}`}>
            {this.listItem}
          </ul>
        </div>
      </div>
    );
  }
}
export default Placeholders;
I used a custom dropdown for adding placeholders. But the essence still stays the same because I use the example from the documentation for a custom button. 
To render the button itself I used the same styling, classes, and structure as is used for the other dropdown buttons. I just switched the anchor tag to div tag and added custom classes for hover style and carrot change. I also used events to toggle classes.
  .placeholder-ul{
    visibility: hidden;
  }
  .placeholder-li:hover {
    background: #F1F1F1;
  }
Lastly, don't forget to import and add a custom button to the editor.
<Editor
   editorState={this.state.editorState}
   onEditorStateChange={this.onEditorStateChange}
   toolbarCustomButtons={[<Placeholders />]}
/>