I recently had a requirement to get selected record id from a main view of an entity i.e. Home page view of an entity and also on a subgrid within the form.

I also needed to pass this ID to my action to do some further operations in the plugin. This is how I did

I have taken reference from  here  and here.

1. Download the managed solution of Ribbon Workbench from here and import it to your Dynamics CRM 365 environment: https://www.develop1.net/public/rwb/ribbonworkbench.aspx
2. Create an unmanaged  solution and add contact entity to it. ( I am taking contact entity as I need to perform action on that, you can select any entity)
3. Create two JPG images web resource with sizes as 16×16 and 32×32 for your ribbon button. You can take the desired image and rescale it online.
4. Create a javascript web resource and below code to it:

   function GetRecordInformation(item)
   {
       var selectedItem = item[0];
      alert("You have Select Record with Id=" + selectedItem.Id + "\nName=" 
   	+ selectedItem.Name + "\nEntity Type Code=" + selectedItem.TypeCode.toString() + "\nEntity=" + selectedItem.TypeName);
   }

5. Publish all your customization and open ribbon workbench tool from your solution area and select the solution that you have created and click ok:
6. Now drag and drop a button to the home grid view:
7. I have named this button as “SELECT” from the button properties you will get once you add and select the button on bottom right side of the ribbon workbench also select the Image for you button by providing the web resources we created in step 3 :
8. Now that out button is ready, we need to add a command and enable rule to it. to do this from the solution component area click on commands and click on + button and then from the right side click on add “Action”  and then click on “Javascript action”:
9. Add your javascript resource by searching in the lookup, if you dont find the script( i faced the issue). Simply add $webresource:WebResourceName for me its $webresource:new_SelectRecord  
10. In the next box – add the function name i.e for me its GetRecordInformation
    
11. Then click on Add parameter and select SelectedControlSelectedItemReferences Parameter. It should look like below:

    

12. The final step is to add an enable rule to this command. To do this – from the solution component area click on Enable Rules and click on + button. On your right side you will be presented with below, Make sure you keep the information as it is:
13. We are done , now click on Publish from the ribbon. wait for it to completed as it might take some time. Once done navigate to your dynamics CRM and the main entity home view, you should see the button:
14. Select a record and click on the button and Bammmm! if everything goes right you will get the record information.

You can do the same thing on a form subgrid, all you have to do is place the button on a form grid instead of home view.

I needed to further processing with the data so I called an action and passed the record id described here : http://sacconsulting.blogspot.com/2017/03/how-to-call-global-action-from.html

I hope this has been helpful!