STEP 1 - CREATE TASK AND MAKE IT OPEN YOUTUBE

• Create a new task called Subs
• Add an "App" action and select YouTube from the list
• Select the "Always Start New Copy" option to make sure the app always starts at the same place

STEP 2 - ADD AUTOINPUT ACTION TO CLICK THE SUBSCRIPTIONS BUTTON

• Add an "AutoInput Action" Tasker action and click the "Easy Setup" option
• Open the YouTube app
• Swipe down on your notification tray and click the "Add" button in the AutoInput notification
• Touch the top middle button in the YouTube app (this is your subscriptions button)
• AutoInput will show you the selected field. Touch the "Accept" button in the notification.
• Go back to Tasker and select the "Subscriptions" option
• Select the "Click" option and accept the action. This action will click the the top middle button in the YouTube app when it is ran while YouTube is opened.

STEP 3 - ADD ACTION TO CLICK THE CAST BUTTON

• Repeat step 2, but select the cast button instead

STEP 4 - AUTOINPUT ACTION TO CLICK CHROMECAST

• Add another "AutoInput Action" Tasker action
• Select the "Click" action
• Select the "Text" field type
• Write "Chromecast" in the Field Text
• Accept and go back out to the task

In my case, my Chromecast is called "Chromecast Work", so I just wrote "Chromecast"

STEP 5 - ASSIGN YOUTUBE ICON TO TASK

• Click the bottom right icon in Tasker. This will allow you to assign an icon to this Task
• Choose "Application Icon"
• Select the YouTube icon

STEP 6 - BACK OUT OF TASKER ADD SHORTCUT AND RUN IT

• Press back multiple times until you exit Tasker. This will allow it to save the task
• Long press your home screen and add the "Task Shortcut" widget
• Select the Subs Task
• Back out of the Task to create the shortcut
• Click the shortcut to run it.

YouTube should open, subscriptions are shown and casting begins.
For more information, check out the AutoInput category page.