Please be aware we've read that this works, but this is not officially supported.
Here is how to set up Lync Online to automatically populate your Speek conferencing number and PIN in every invitation:
1. From your Office 365 Admin Dashboard, click on the “Service Settings” link.
2. On the “Service Settings” page, click the “IM, meetings, and conferencing” on the left side.
3. On the right side, which just loaded, under the “Dial-in Conferencing” section at the top, click “Manage” (or configure).
4. Go through the setup process, but here is the trick – when it asks for a conferencing provider, select ANY of them and continue through.
5. When it asks for the “phone number” and “passcode” you received from the conferencing provider, enter your Speek phone number and access PIN (from your dashboard). Click save.
6. If necessary, add these numbers to each person in your organization as needed.
Now, you can go to your Outlook Calendar, set up an event and click on the “Online Meeting” link about halfway down the page. After a brief pause, you’ll see your Lync access link and your Speek number with the passcode in the body of the event invite. Invite your attendees and send away!