How to check a checkbox without unchecking?

I am trying to make a script which somehow will check a checkbox (but not uncheck it if it is already checked). Is there a button / shortcut which will check a checkbox? (Clicking will uncheck it if it is checked, so will tab + space) Is there a way of only checking it (does nothing if it is already checked)

I hope this is clear enough, I need a way of checking a checkbox which won't uncheck it if it is already checked.