Hello. 
  I can't login to the Salesforce App, using my trailhead instance... 

- The iPhone app gives me two options:  Production and Sandbox.  
- I have the option of logging into these (or adding a new account) + selecting an alternate domain. 
 -- The format for adding an alternate domain is stated:  https://domain.my.salesforce.com. 
However, when using trailheads (e.g., like this one: Create Global Quick Actions Unit | Salesforce Trailhead
https://trailhead.salesforce.com/trails/force_com_dev_beginner/modules/salesforce1_mobile_app/units/salesforce1_mobile_app_actions_global
)
... I use a trailhead instance, where the URL is:  https://brave-raccoon-000000-dev-ed.lightning.force.com
 {where the 000000 is my trailhead domain number, which I masked for this question} 

How do I log into Salesforce App with my trailhead instance? 

Thanks
 Jarrell