I don't like the command=argument syntax, since you usually need to use it twice - I suppose once to define the argument and once to execute the command, regardless of how many commands there are. That uses up CPU cycles for nothing. Therefore, I much prefer to execute commands directly.
However, both with the direct and the indirect way, I would expect that you will need to enclose the argument in parenthesis where necessary, for example because of the : and / letters in URLs. I suppose that it is why your first try did not work.
So you would need set your OnPress to
browser "http://eq2interface.com"