HI @mustafghan,
You can do this:
library(httr)
GET("http://google.com/", path = "search", query = list(q = "green eggs and ham"))
So, I assume that you could do something like this:
GET("https://api.census.gov/data/timeseries/intltrade/exports/hs?get=CTY_CODE,CTY_NAME,ALL_VAL_MO",
query = list(key = "my_autho_key",
YEAR = "2017",
MONTH = "04"))
But, of course, I can't test it.
HTH