In step 11 of chapter 5.3, "Creating hosts with unattended provisioning" of Satellite 6.15's Provisioning Hosts guide, one is instructed:

Click the Parameters tab, and ensure that a parameter exists that provides an activation key. If not, add an activation key. 

This is too vague to be useful.  Because the parameter's name field is free-form, one needs to know the exact string to enter.  I had to refer back to Satellite 6.2's Provisioning Guide to find out that I had to use "kt_activation_keys" for the name.