Showing a simple way of populating an SPFX dropdown with code. See http://cob-sp.com/SPFx-WP-Props2

SimpleDynamicDropdown.ts

private fetchOptionsSimple(): Array<IPropertyPaneDropdownOption> { 
    var options: Array<IPropertyPaneDropdownOption> = new Array<IPropertyPaneDropdownOption>(); 
    options.push( { key: 'Added1', text: 'Added from code 1' }); 
    options.push( { key: 'Added2', text: 'Added from code 2' });
    return options; 
  }

  protected getPropertyPaneConfiguration(): IPropertyPaneConfiguration { 
    var fetchedOptions: Array<IPropertyPaneDropdownOption> = this.fetchOptions();
    
    // other code for property pane here..
  }