Web scratching and push data

I want to connect to a site as I am an agent and it looks like they don't have any api. Another website has manage to interact with the site in question and pull information from the site, but also push information in order to do bookings. To access the site there is a login. I wonder if it is possible to push information to a form hosted on the site in question without an api. If not then the site must have an api, only that they don't know. Its very important to me to find out how this can be done. Thank in advance.