There is probably a more elegant way to do this but I used a series of expressions to achieve this. This would really only work if you have a finite amount of labels that you have already predefined.
I have a planner that has three labels for Small, Medium and Large. The expression checks if the label exists (is True) and writes my label name to the field. Create an expression for each label.
if(equals(triggerOutputs()?['body/appliedCategories/category17'],True),'Small','')
if(equals(triggerOutputs()?['body/appliedCategories/category15'],True),'Medium','')
if(equals(triggerOutputs()?['body/appliedCategories/category13'],True),'Large','')
triggerOutputs()?['body/appliedCategories/category17'] is the "Light green" label which has been renamed as "Small" in my planner. 