Option 1 (Recommended): Use E2CP's Invocable Method

Email to Case Premium packages an Invocable Method action, "E2CP Send Response," that replicates what happens when sending from the New Comment Page: comment-only sends (no email), public/private comments, canned responses, templates, file attachments, drafts, and scheduled sends. It handles both the "comment only" and "comment + email" scenarios in a single supported call, and can be used from Flow or Apex.

For setup and sample code, see:

• Invocable Method - Sample use cases with Flow
• Using E2CP's Invocable Method in Apex
   

Option 2: Insert a Case Comment and Email Message directly

If you have a reason to bypass the Invocable Method (for example, replicating exact legacy behavior, or an edge case it doesn't support), you can insert the underlying records directly. Both records must be created in the same transaction so Timeline's timing-based pairing associates them correctly.

Please use the following when creating your local APEX:

insert new CaseComment(
ParentId = {case id},
CommentBody = {plaintext body},
IsPublished = true
);

insert new EmailMessage(
ParentId = {case id},
FromAddress = {from address for the outbound email},
FromName = {display name for the outbound email},
ToAddress = {email address(es) sent to directly},
CcAddress = {email address(es) sent to as a CC},
BccAddress = {email address(es) sent to as a BCC},
Subject = {email subject},
TextBody = {plaintext body},
HTMLBody = {html body},
MessageDate = System.now(),
Incoming = false,
IsClientManaged = true,
Status = '3'
);

 
For an example using Flow, see:

• Add a comment in Timeline after a support agent takes ownership of a Case.