Create an extrusion parallel to the axis of the cylinder and use edges from the cylinder and the rectangle as references for the sketch. If the cylinder touches the rectangle, then you will need to offset the edge of the rectangle so that there is not a point of contact that short circuits the sketch border. Alternatively sketch a rectangle that intersects the cylinder, but not the hole.

Looks like you are trying to extrude toward the cylinder. You could extrude up to the inside hole but I think that is going to give you a small step between the cylinder and the rectangle. You could extrude up to the center of the cylinder and add the inside hole later. You can extrude in a different direction so you can use the cylinder as an edge in your sketch. Lots of ways to make anything but you do need to think it through and plan ahead.

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Is the rectangular part thickness the sane as the OD of the cylinder?
Is the length of the cylinder the same as the width of the plate?

If yes, then make a different sketch that was the plate with a radius on the end and then add the hole as a secondary feature.

If the plate is longer or shorter than the cylinder, use the edge of the cylinder and match the plate end to the radius, extrude in the direction of the hole.

Extruding the plate to next surface will give you what you see since the outer cylinder boundary is the next surface.

You need to think through the possibilities of what will happen before you just start sketching.

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