Each teacher has one student account to preview how it looks like from a student's perspective.
After logging in to the teacher's account, click the profile photo found at the upper right corner > select 'Switch to student account' to enter your demo student account.
To assign homework to your own student account, look for the account in "Tr-Stu" class in this account level (e.g. '3-TrStu').
In your demo student account, you can always click the Switch to Teacher button at the bottom left to return to your teacher account.