how to bypass form filling of asp form with direct http request

As i guess, filling an asp form will finally pack all data filled and make a post request to a certain url to update the data. How could i bypass the form filling efforts and pass the required values with other external methods, e.g via making http request. How could i investigate what have to pass and what the destination url of the form is.

I have tried to look into the html scripts but could only know the web will trigger a javascript when pressing the button. However, i have no clue on the next step.

Thanks!!