If you want a iambic pentameter (penta for five), you cannot treat a word that has two syllables as having one syllable just to make it "match".
Moving away from the regular meter is something poets do. Still, for a homework that is supposed to make you think about the strict form of the sonnet, I would suggest you rephrase the line to stick to the rules.
"I long for you to send the healing rain", maybe?
