In some cases, you may need to link two content folders instead of using a drop-down selection. For example, the blog category. 

It's better to create a content type and folder called 'Category', so that we can add a new category easily by just create a new content under that folder.

Create a text field and add some categories.

To link two content folders, click Contents - Contents on the main navigation menu. Click the 'Setting' button on the right of the content folder that you want to link. In this case, we select the blog article.

In the popup window, switch to the relation folders tab, click '+' button to add a folder.

Select the folder you want, set the alias and click 'Save'.

Now when you add a new blog article, there will be a field to let you pick the category.