1998 f150 will not start, is it the primary batter fuse?

May have blown the fusible link wire, on starter/solenoid (if it blows you have 0 power anywhere). If you have a volt-ohm meter, you can check for voltage along that cable, put the black test lead on the neg. batt. cable, and check with the red test lead along the batt. cable and smaller wire on the solenoid, you should be reading 12v. If you blew the fusible link where you need to figure out why, a dead short most likely, you need to fix that otherwise you'll keep blowing it.

I'm going from memory here, as I owned an older F150, you may have the starter relay mounted to the fender wall and not a solenoid on the starter, in which case, near the battery you'll see the relay, a big fat thing with 3 or 4 terminals on it and several wires connected to it, one fat cable runs to the starter, one or more of the smaller wires are fusible links, they protect the wiring harness from being damaged in case of shorts or excessive voltage. Use the meter to check these wires.