A new UITopPage will by default display the project's home page on startup (after the logon has occurred).  To select a Workflow Container Web page set as the first page, add a UIWorkflow element to the top page:

This will add a new, unnamed Workflow Container page to your UITopPage, which you can immediately rename in the Properties:

Refer to Modeling a Workflow Container Page for information on how to further design the new Workflow page.