Zenkit

You can add a label to an item really quickly by adding a # followed by the label name into the primary field (the 'name' or 'title' of the item).

For example, when you add a task to your Spaceship Build collection, you could type "Develop spacewalk safety line #Todo #high". This will automatically add the labels 'to do' and 'high' priority to the task.
Make sure you don't include spaces or special characters while typing in label names, otherwise Zenkit won't recognize them. For example, #to-do won't apply the label 'To-do'.

You can also use this trick to change a label that's already applied (if you only allow one label from that field to be used), or just add another label when the field allows multiple labels.