Shalom, Eltanin.
Sure. The write-in box or window is a place on the ballot where one can write the name of the candidate of your choice, if you want someone other than those of the two majority parties and all the minority parties. It's usually the last line of the selections available. You will notice, too, that there are other candidates than those of the two majority parties. We DO have alternate, minority parties that like to throw their hats into the ring. That is your prerogative.
HOWEVER, such write-ins are seldom successful. Many who use the write-in option do so to make a statement. They will write in "Mickey Mouse" or "Charlie Brown" or even "Snoopy" to say that ANY CHARACTER would be better than the majority candidates! I used to work in a polling district as one of the voting machine custodians. It was our job, on occasion, to perform the recounts manually, and I've seen some real DOOZIES written in! "Paul Bunyan," "Superman," "Gene Wilder," "Hannibal Smith," "Mr. T," "J. R. Ewing," "Eliot Ness," "Captain James T. Kirk," "Big Foot," you name it! Many have chosen their OWN names; some put in a relative's name; some put in a teacher's name. The only problem is that it is basically a non-vote.
Now, if a political group was large enough and could convince enough people to vote for ONE particular candidate, it might be possible to truly make a difference, but it is unlikely, primarily because of the Electoral College. Who you are REALLY voting for is the electoral member who will vote on behalf of his/her constituents for a candidate, and he or she will most likely vote for the majority-party choice of his constituency. Ah, America! Who else in this world has so twisted a system?