Here’s how you can create Flyout menu in Ribbon Workbench.
Also, for most ribbon customization, you can edit using the Command Editor in Power Apps but this is in Preview at the time of writing this post –

1. How to add low-code Power Fx buttons to model-driven apps (Commanding v2) – https://www.develop1.net/public/post/2021/07/25/commandingv2
   
2. Comparison between Ribbon Workbench and Power Fx Command Buttons –
   https://www.develop1.net/public/post/2021/07/25/RibbonWorkbench-vs-PowerFx

Meanwhile, here’s how you can create a Flyout menu in Dynamics 365 CRM Ribbon using Ribbon Workbench in the XrmToolBox

Ribbon Workbench in XrmToolBox

In classic XrmToolBox style, here’s how you create a Flyout button. Given that you are aware of how to load the Entity’s ribbon in a solution and then onto the Ribbon Workbench, we’ll see at how we can create a button on Account Form –

1. Insert the Flyout menu in the Ribbon where you want to insert this. In this case, in the Form ribbon of the Account entity.
   
   
2. Once dragged, pick a Menu Section control and insert into the flyout canvas shown in screenshot below. Because all the Buttons will fall under that Buttons can’t be added directly into the Flyout menu.
   
   
3. Now, you can name the Menu Section itself as show below –
   Note: In case if you named the Section correctly and it didn’t appear the first time, clicking on some other component and then clicking it back will show it correctly.
   
   
4. Now, you can insert buttons underneath the section. In this case, I’ll add 2 buttons for Pause and Cancel.
   
   
   
5. Make sure you populate the Command and the ModernImage icon for it’s icon so that the buttons are visible when you publish. Without a Command being attached to the Button, the button will not be visible.
   The final button layout will look like this.
   Note: In case if you named the Section correctly and it didn’t appear the first time, clicking on some other component and then clicking it back will show it correctly.
   
   
6. Once ready, publish the changes and see. Your Flyout menu will be ready and will look like this!
   
   

Hope this was useful!

Here are some more XrmToolBox related posts which you might want to check –

1. Show custom ribbon button based on Security Role of the logged in User in Dynamics 365 | Ribbon Workbench in XrmToolbox
2. Connecting XrmToolBox to an MFA enabled Dynamics 365 environment | Azure AD
3. Find deprecated JS code used in your Dynamics 365 environment | Dynamics 365 v9 JS Validator tool | XrmToolBox
4. Delete App Passwords created by other users in Office 365 | Multi-factor authentication
5. Single record and multiple record auditing in Dynamics 365 to Audit record access | M365 Compliance
6. Filter records in a View owned by a Team you are a member of | Dynamics 365 CRM
7. Show custom ribbon button based on Security Role of the logged in User in Dynamics 365 | Ribbon Workbench in XrmToolbox
8. Ribbon button visibility based on a field value in Dynamics 365 | Ribbon Workbench
9. Get GUID of the current View in Dynamics 365 CRM JS from ribbon button | Ribbon Workbench
10. Pass Execution Context to JS Script function as a parameter from a Ribbon button in Dynamics 365 | Ribbon Workbench

Thank you!