I don't know about your IP core but can talk cofdm theory:
Your vector is 4096 points, Your Fs = 100MHz , your target is that first point on this grid(frequency 1 after dc) = 12.207KHz
All you have to do is think of an input that has a grid of 8192 in the first place. Then you set this vector to values for amplitude at all frequencies up to 50MHz:
(100MHz * 1/8192 = 12.207KHz).
(100MHz * 4096/8192 = 50MHz).
If you are constructing the vector then yes you first think of 4096 then make amplitudes beyond 4096 a reflection of 1:4096 as below
vector_8192 = [1:4096 4096:1]
Though strictly speaking I believe, the dc value should not be repeated at reflection.