"Throdamas come from the portal on the Battlefield. You will see a moon selector at the top of your window when you take the Portal zone. Switch it to Ganymede to get a Throdama instead of a Symurgh. Only Worker Throdamas will join you; specialized morphs are bound to their home hives on Ganymede and won't follow the colonist Workers."

I saw this text in the latest news post, but no matter how much I look I can't find a moon selector.  Could someone provide me with a screenshot or help with this?

Thank you in advance!

You can't see the moon selector because you aren't on the portal on the tactic map.  The portal is the middlish zone on the tactic map.  Also, to get on the battlefield and hence to the portal is to build a military academy and then make a squad. 

Feel free to message me for more help!