Can you really insert the text you want in one-time pad?
It is quite simple and stems from the idea that flipping one bit in the ciphertext flips the corresponding bit in the plaintext.

So, say the ciphertext is $1011$ and we know the plaintext is $0101$ (thus the key is $1110$). Say we want a plaintext of $0000$, we just have to change the ciphertext to $1110$ (notice where the bits have been flipped) and we have $1110\oplus 1110=0000$

In that sense you if you can flip any bits you want and you know what the plaintext says and you know what you want the plaintext to say, you flip the right bits in the ciphertext and decryption comes out as you desired.