If it is not a load-bearing wall, you can do this:
Insert a standing stud (145*45) horizontally halfway into the vertical ones before cutting them. Below this stud, you should have prepared reinforcements on each side of the opening to support this beam. All reinforcement is done with 145*45 studs. You can then choose to have additional reinforcement upwards by inserting another stud on the other side.
Hope this made some sense.
I also recommend that you contact a structural engineer for on-site advice and, of course, the landlord if you haven't already done so. Also, search the forum for similar threads.
/CC