I don't know OpenTK, but defining a quad via two points is easy:

x1,y1.......??,??
......................
......................
......................
??,??.......x2,y2

The two missing points are defined as:
x2,y1 and
x1,y2.

So the quad, in CCW order would be:
x1,y1,
x1,y2,
x2,y2,
x2,y1.

If you only know the one set of coordinates and the width and height,
x2 = x1 + width
y2 = y1 + height.

Someone else will have to show you how to translate that into OpenTK commands, but the above will be universal to any rendering API.

I don't know OpenTK, but defining a quad via two points is easy:

x1,y1.......??,??
......................
......................
......................
??,??.......x2,y2

The two missing points are defined as:
x2,y1 and
x1,y2.

So the quad, in CCW order would be:
x1,y1,
x1,y2,
x2,y2,
x2,y1.

If you only know the one set of coordinates and the width and height,
x2 = x1 + width
y2 = y1 + height.

Someone else will have to show you how to translate that into OpenTK commands, but the above will be universal to any rendering API.

I don't know OpenTK, but defining a quad via two points is easy:

x1,y1.......??,??
......................
......................
......................
??,??.......x2,y2

The two missing points are defined as:
x2,y1 and
x1,y2.

So the quad, in CCW order would be:
x1,y1,
x1,y2,
x2,y2,
x2,y1.

If you only know the one set of coordinates and the width and height,
x2 = x1 + width
y2 = y1 + height.

Someone else will have to show you how to translate that into OpenTK commands, but this will be universal to any rendering API.

I don't know OpenTK, but defining a quad via two points is easy:

x1,y1.......??,??
......................
......................
......................
??,??.......x2,y2

The two missing points are defined as:
x2,y1 and
x1,y2.

So the quad, in CCW order would be:
x1,y1,
x1,y2,
x2,y2,
x2,y1.