Design a Wireless Antenna at PCB

Good day~

I am design a PCB for a wireless Thermometer by using the RFM69CW Radio Module and the ATMega328p as an MCU. And I don't know how to deal with the antenna when I placed the MCU. My situation is as following:

While prototyping the setup on a breadboard i simply soldered a 1/4 wavelength (865 MHz) monopole wire antenna to the antenna pad of the RFM69CW modules and everything worked fine. However now I read a lot about RF Design and I am completely unsure what I should or shouldn't do.

I originally intended to simply route a standard trace from the ANT pad of the module to a plated trough hole and then solder my wire antenna to the hole. However, I read in several places, that i have to match the impedance of the trace to 50 Ohm and that i also have to put a ground plane next to or under(?) the trace.What is the best way to add a simple wire antenna to my PCB design? I already saw designs like the Aaduino or the Moteino from LowPowerLabs that just connected a plated trough hole some millimeters away from the ANT pad of the RFM69CW and put a ground plane on the other side of the PCB. Is it that simple or are there certain calculations behind these designs?

Please see the above picture: it's my design, the trough hole on the top left is my current attempt for mounting the wire antenna.

I will appreciated it if you could give me some suggestions.