Go to navigation, in the properties dock you see a property 'home'. You can change this property by clicking the '...' button. You are able to select a form as startpage.
Since 2.4.3 you can also select a microflow as 'startpage'!
Besides configuring the desktop screen in the Modeler, it's also possible to make an own customized page by (re)placing the file "welcome.mxf" in the directory "web/forms/locale" (e.g. web/forms/nl_NL). This can be useful when you would like it to fit a particular theme. Please make sure the Modeler has no form configured as home page when you do this, otherwise your customized page will be overwritten when generating forms (the "Synchronize modeler file" option when the server has been started).
This is possible:
I hope this answers your question.