Welcome to the Community!  Will see you on the FB group too.

Your topic says "Adding forms to print with returns" but your question is about engagement letter, which happens before a return is prepared.

To customize your engagement letter template, simply follow these steps:

1. Click on "Intuit Link" in the left panel of PTO;
2. Click on Settings on the top right corner, next to Help and the Create request button;
3. Scroll down to the Organizer templates section:
   1. Click on the small triangle next to Default Personal Tax Organizer to expand the items.  On the line for Engagement Letter, click on Edit to customize the default letter as you wish; or
   2. Click on + Create a template above Default Personal Tax Organizer to create a brand new set of organizer template and build a totally new Engagement Letter of your own.

If you would like to include a file (e.g. PDF or Word document) as an attachment or addendum to the Engagement Letter in Link, your only other options are:

1. Within the Link drawer of PTO:
   1. Create a new item under the Document Checklist of a particular client;
   2. Get out of that item;
   3. Cick on the Act as taxpayer checkbox in the bottom left corner of the Link drawer; and
   4. Click on the Upload a Document button under that item you just created; or
2. Open the Link for Accountant portal (which was deactivate for TY2018 but apparently reactivated for TY2019) at https://link.intuit.com/accountant but note that this can only be done after that client has accepted your invitation to Link;
   1. Select Tax Year 2019 from the dropbox on left panel under the Link logo on the top;
   2. On the line with your client's name, click on Review under the ACTIONS column on the far right;
   3. Click on the Add Request button on the right;
   4. Under Single Request, select Engagement Letter as the Request Type;
   5. Type in whatever basic instructions/descriptions you would like your client to note about the document under Request Text;
   6. Click on the Attach file(s) button to select the file(s) to upload;
   7. Click on the Save button; and
   8. If you are satisfied with everything, click on one of the buttons in the You're almost there box.

Hope this helps.

BTW, if you have not used Link before, it would be advisable that you set yourself up as a mock client and test out the process flow on both ends as the accountant and client, so that you are aware of some of the quirks and can answer your clients' questions as well as problem solve for them when they run into problems.