How do you expect Anjali to help you? Since you have said that you both are close to each other, all you need to do is to tell her that you have sex on your mind and want to have sex with her. If she too has the same thing on her mind that you have everything worked out.
But let me warn you that if you are planning to drug her and then have sex with her, that would be criminal in nature and would get you into lots of trouble, even a jail sentence.
Learn to control your libido. Travelling on a bike together is not an indication of her closeness to you. Remember you cannot keep a distance on such mode of transport. Since you are not a stranger, she may be changing her clothes in front of you, but then that again is not an indication of her sexual desire. It is however disgraceful of you peeping through the keyhole when she is having a bath. Imagine the consequences if you are caught with your pants down watching her bathe and she has no intention of showing herself nude to you.
What makes you think we believe you when you say that you want to have sex with her only once. If it is with force and deceit it may be only once, but if you have her consent it can be over and over again. Decide what you want.