How to add drop down menu for Custom fields attributes?

Step 1:

Click on the Asset Types navigation bar item, found on the left beneath the Job Types navigation bar item.

This will take you to the below screen.

You can also search for an Asset Type that needs to be edited by using the search function, as shown below.

Step 2:

Click on the Asset Type and you will be taken to the Overview screen, which shows asset summary information as shown in the below screen.

Click on the Custom Fields menu item to add custom fields with a default attribute.

 Step 3:

On a new Custom Field or existing one, tap on the Custom field to add a menu to and click on the Predefined Values button, it should show (0) since no values have been added yet. 

The Predefined Values Editor screen will pop up.

Under the Add New Value, enter the first value then press Enter or click on the +button.

Add the next value, until complete.

You can set the Default value if desired.

Click on the Save Predefined Values button to save.

Note that the Predefined Values button is now showing the number of items on the menu.

Tap on the Save button.

 On the mobile app, the Custom fields will look like the below.