Filter A LookUp SelectBox by a Single Line Text

Hi Guys 

I have tried a few things but I seem to be getting nowhere.

I simply want to filter this LookUp SelectBox (Subcategory) dependant value in the sys_choice table by the Single Line Text field (Category Label)

Bit of context: I was having a nightmare trying to filter directly via the other LookUp SelectBox (Category) but when I did A script include and hard coded the Category as 'End User Hardware' the filter worked!
So now I'm trying to just simply filter by a string.

I am currently just seeing if the following reference qualifier will work:

javascript:
var label = current.variables.category_label;
'inactive=false^element=subcategory^sys_domain=global^name=incident^dependent_value=' + label + '^ORDERBYlabel';

Any help welcome