C# OpenTK game AND winform windows

I want to have a separate winforms window that has a bunch of GUI elements like buttons, sliders, textboxes, etc that will be used (non-modal) while the rendering loop is happening in the fullscreen rendering window. I tried adding a blank form and it won't even let me get past "Form1 f = new Form1()" because it doesn't recognize Form1 as a class.

I googled a bunch but I could only find things pertaining to using the control, nothing with the fullscreen mode. Is this even a possibility with OpenTK?