This plugin uses the One Call API from OpenWeatherMap.org.
In your ~/.bashrc add the following definition and place your APIKEY in there
# waybar weather settings:
export WAYBAR_WEATHER_APIKEY="<YOUR OpenWeatherMap API KEY>"
export WAYBAR_WEATHER_LAT="44.43"
export WAYBAR_WEATHER_LON="26.02"
export WAYBAR_WEATHER_UNITS="metric"
export WAYBAR_WEATHER_EXCLUDE="minutely,hourly,daily"