There are many spells, in traditional grimoires, to achieve effects such as the one you are asking.
For example, and this is a very well known one, with many variations. Write in a piece of parchment your full name and the one of the one you want to get into their mind with your blood (if this turns out to be a heroic act, I guess it'll also work with your initials, having the extra advantage of, in case of this being discovered, and so the spell broken, you won't be exposed.) Then take an apple, cut it in half and place the parchment inside. Put the apple back together and use some twings of an apple tree to hold it in it's place. Now sneak into his/her bedroom and hide the apple near his/her bed. The closer to the pillow, the better.
This is but one example of a traditional spell...you can also use the power of the Moon, which rules over dreams. For example, you can charge a piece of quartz or another crystal by exposing it to the full moon (some purple candles (9 is the number of the moon) and a nice Lunar Invocation are also nice, of course, stating the intention of the spell) and giving the crystal to him/her, or placing it near his/her pillow. If you use blood or sexual fluids to charge the crystal, it will increase the potency of the spell.
These are two spells, one using traditional magic, and the other, Lunar magic.
If you do try any of these spells, I wish you good luck
A spell that I was taught when I was younger was to take a hair from the person's head, and burn it with a slip of paper with the intended dream content. It was then kept in a black bag or box with an amethyst crystal.
Good luck with your endeavors!