Alter Rectangle Polygon by Keyboard to Have Specified Dimensions (eg 1km x 1km)

Here is my current workflow.

1. Press "New Shapefile Layer" with "Geometry type": "Polygon"
2. Press "Toggle Editing"
3. Press "Add Rectangle from Center and a Point" then draw the rectangle to the current extend

Now I’d like to alter the current dimensions by keyboard to 1km by 1km for example. But I don’t know how. Specifying the dimensions on creation of the polygon would also be totally fine as well.

How to create a polygon with specified dimensions? and How to create polygons with straight lines and right angles in QGIS? looked promising but actually don’t give an answer.