Code for username and pw button for login validation from website

I'm just starting to learn coding. What I'm trying to do in visual studio 17, is tie username button and password button that logins by validating through a website http or https etc. I'm really not sure where to begin. My username and password button code is basically blank and I can't seem to find any real videos or information or code for this exactly. The website used does have registered users but as long as the username and password match what they use on the website it should let them access the application or launcher. Thanks in advance for any help or hopefully the solution and code.