Set criteria for a checkbox field in a view with URL

Set criteria for a checkbox field in a view with URL

I have a checkbox field. I need to filter a view depending on a value of a selected checkbox field. For example, view all records where option1 is selected. Which operator should I use?

I tried 

URL?fieldname=option1 

and 

URL?fieldname=option1&fieldname_op=26 

- none of them worked. 

Help please!