If you want to use an asynchronous message in Software Ideas Modeler, you can follow these steps:
Choose Call Action Tool
Choose Call Action tool from toolbox, then press the left mouse button on the caller lifeline (Lifeline1), hold the button and move to the called lifeline (Lifeline2) and release the button - the call message between Lifeline1 and Lifeline2 will be inserted.
Set Call Action Name
Enter the name of call action.
Open Properties Dialog
Click using right button on arrow or name of call action to show the context menu and then click on Properties menu item (or you can alternatively double click on the call action) to open the Properties dialog.
Set As Asynchronous
In Properties dialog open the Message tag and check the Asynchronous check box. Your setting confirm using OK button.
Asynchronous Call Message
And here is the result: