Is TF command the "Staff Officers" option in the Leaders tab? When I choose that, no positions appear under possible assignments. Is there a tech I have to research?
Yes, your Task Force command is under Staff Officers.
1a) Select a rank of naval officer to choose from on the left -or-
1b) Select a set of criteria from the bottom-right
2) Select an officer from the relevant pane to your previous method (bottom left or bottom right respectively)
3) Select 'Staff Officers' from the dropdown in the middle
You should now have a single position available to assign, "R3 Fleet Headquarters" (note: this is the name of the Task Force, so if you set up a different Task Force for some reason you'll get that)
(Note that you must have an officer selected for an assignment to appear, for some reason)
4) Assign them to the position with Assign.
You now have a Task Force Command position assigned; that's the leader of the Task Force. There are then several other staff officer positions that become available, such as Logistics Officer, Public Relations Officer, etc.
One catch is that these positions are set to require one rank below the TF Command as a (edit to prevent misinformation) maximum
minimum - so if you put an R4 in the Command slot (or your R3 gets promoted), you can now only assign R3s or above as staff under them. If your TF Command gets promoted existing positions remain, though, so you can keep R2s in their Staff Officer role even if TF Command is an R8 - as long as they were assigned at an appropriate time.
This is obviously a non-issue if you have realistic promotions turned off so you can manage them manually.