You would enter the command in the 'command to run' box
0 7 * * * wget http://www.domain.com/file.php
Then select from the drop down next to 'hours' the option 'selected hours'.
Then in the next dropdown that appears you would select the hour required e.g. '07'
The other options will show every day, every month, every day of the week unless you wish to change them.
When you select 'ADD' it will create the crontab line you want.
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I am NOT a DreamHost employee OK! :@
Act on my advice at your own risk!