You can start an Online meeting instantly via the New > Online meeting option.

However, to plan one in advance, you need to use the New > Event option instead.
On the first run you can decide whether you want to create a standard event or online meeting.
You can also select a default for future cases or just pick each

You can of course then choose which Online meeting provider should be user or even change your mind about the event type while creating the event itself.

Read more in our blog: How to set Online Meetings in eM Client