You can add a new table variable to your KW and fill it with values. Then, in KW itself, give the condition "if then" where to check the value of the parameter "URL" If no url is specified, take a value from the variable as default value.
I myself often run KW tests with default parameters and there are different solutions for this. I'm just offering a one of ideas.
It looks like there indeed was a misunderstanding from our side - my apologies for that.
As for this particular task, it looks like you have found a solution that works best for you. Just as a theoretical alternative, I can also suggest using the project variable in the keyword test directly so that the parametrization only takes place once.