If user wants the ability to create a new time entry without leaving the Revenue Grid email sidebar, then a system administrator will need to create a custom action.

How can I create a new time entry directly from the Revenue Grid email sidebar?

The Revenue Grid email sidebar is not lightning ready, so a classic action will need to be created.  Go to Setup >> Object Manager >> Matter >> Buttons, Links, and Actions.  Select the New Button or Link button on the top right.  Complete the fields as per the instructions below.

1. Choose a name for the new action. (for example: New Time)
2. Select tab to auto-complete this field.
3. [Optional] Add a description of what the action does and where it is used.
4. Detail Page Button
5. Display existing window with out sidebar or header
6. URL
7. Use one of the two options below for the formula:
   {!URLFOR($Action.advpm__Time__c.New, null, [qsid= advpm__Matter__c.Id ,retURL= advpm__Matter__c.Id], false)}
   or
   {!URLFOR($Site.Prefix+'/apex/advpm__TimeEdit', null, [qsid=advpm__Matter__c.Id,retUrl=advpm__Matter__c.Id], false)}
8. Select this button to check your syntax.