Filling in fields on an external web page and clicking button

I have a need to build a form that will collect data and then send that data to a 3rd party web site but it is not something i have done before.  Can anyone point me in a direction?

Basically, i need to fill in some fields from this external web page and then cause a button on the external page to be clicked....but the page is not mine and i have no direct access to it.  My only access is via http.  So, how do i put the data into the fields on the page and cause the button to be clicked?

 

Thanks,
Majorcats
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