I believe Gebo is number 1 here for love and energy exchange, with Ehwaz being also close, but more Mercurial. Wunjo is related as it patterns to joy, but not as direct as the others.
Regarding sexual connection, I would use Kenaz, and I would do this as a separate working, like on the upcoming Scorpio esbat.
Use either the number of the runes themselves, or other power numbers, like 10, 40, or here you can use 5 or 8, as this pertains to love and power, respectively, which I believe includes sexual power.
Regarding affirmations, you basically already made this when you stated you goal to "empower and make a relationship".
Example : [Chant then visualize the energy's action] then state something like "The romantic relationship bonds between us have been permanently created and improved, in the best way for both of us" x5