How can I change the type of the message in a sequence diagram from synchronous to asynchronous while modelling it with Sparx Systems Enterprise Architect 15.0?

I recall in the past it was just either an obvious option in the context menu or property or both. The official Sparx documentation (for current 15.1 version) suggests the same:

Create a Message on a Sequence diagram

Option: Synch
Click on the drop-down arrow and select 'Synchronous' or 'Asynchronous' as appropriate.

The value 'Synchronous' disables the 'Kind' field; synchronous Messages are always Calls.

Yet I can neither see it in the context menu nor in the Properties anymore.

The only way I managed to do it is by changing the default message type for the sequence diagram, but changing it each time to set an asynchronous message (and changing back to synchronous) seems an overkill.

Neither Google search nor search on SO gave me any reasonable suggestions. It has allowed me to find a workaround mentioned above only.

Note, I am not talking here about IsReturn, this works fine. I am talking about an asynchronous message, represented with a solid line and an open arrow.

The answer is rather simple. You are not showing the whole properties view.

If you drag the bottom a bit further down you'll see the option to set the synchronous/asynchronous