How to get a file downloaded from an .aspx website after completing a form

Im trying to get a file downloaded from the website posted below after completing some fields.

the fields i would like to complete are:

  1. the dates in "Fecha Declaración en SioGranos:" "01/08/2018" to "31/01/2018"
  2. product name in "Producto:" that should be equal to "SOJA"

Adding here what I did so far but couldn´t even construct the url in the correct form.

txtFechaOperacionDesde<- "01-08-2018"
txtFechaOperacionHasta<- "31-08-2018"
ddlProducto<- "SOJA"
uid <- 4
res <- read.csv(url(csv))

Created on 2018-08-06 by the reprex package (v0.2.0).

tried with rvest but i could set a few parameters only.

any help is appreciated.

Hi Pablo - do you mind posting this as a REPRoducible EXample (reprex)? A reprex makes it much easier for others to understand your issue and figure out how to help.

thanks, I have just added what i have so far.