Your "username" is going to be the name of the record that has been tagged dynamic. ie You marked the A record for dyn.example.com as dynamic. Your username will be "dyn.example.com"
You can tag an A or AAAA record by editing it once you have selected the zone. (check the box).
Once you have "activated" the record to be dynamic, you will need to generate a key (or password if you prefer) for it. (click on the generate icon) to generate the key for the dynamic record.