Welcome!
What macOS version are you running? OS X10.11 as shown in your profile or something else? That may make a difference.
In at least 10.14 Mojave and older, you can instruct TM to backup external drives by doing system Preferences > Time Machine and clicking the "Options..." button. That brings up this window:

Us the "+" button to add drives. NOTE: the drives must be mounted before doing this.
👉🏻 HOWEVER....I must give you a caveat:
I generally recommend against backing up externals unless they are very fast externals with 2000+MB/sec transfer rates. Most regular (cheap) externals people have are dreadfully slow (50-150MB/sec) and it will take a long time for TM to do even its normally fast incremental backups. That can cause system stutters that could interfere with your workflow.
Your decision.