First, you will need to make a button in your C# Form.
Then, double click the button and put the following code: System.Diagnostics.Process.Start($"fivem://connect/{IP}:{PORT}");
{IP} stands for your FiveM server IP Address and {PORT} stands for your FiveM server port.
I'm not going to upload a source code, because it's really easy to make, the design is a bit harder tho.