SPFX propertyPaneSettings() code for a PropertyPaneDropdown populated dynamically. See http://cob-sp.com/SPFx-WP-Props2

WebPartPropetyPane_DynamicDropdown.ts

protected getPropertyPaneConfiguration(): IPropertyPaneConfiguration {
  if (!this.listsFetched) {
      this.fetchOptions().then((response) => {
        this.dropdownOptions = response;
        this.listsFetched = true;
        // now refresh the property pane, now that the promise has been resolved..
        this.onDispose();
      });
   }
  
  return {
    pages: [
      {
        header: {
          description: "Basic settings"
        },
        groups: [
            {
            groupName: "COB dropdown field (PropertyPaneDropdown)",
            groupFields: [
              PropertyPaneDropdown('dropdownProperty', {
                label: 'This is the label',
                options: this.dropdownOptions
              })
            ]
          }
        ]
      }
    ]
  }
 }