There are many ways to achieve this.
I would use the parallel line tool. On the top is the tool option bar, where you can enter the distance for the parallel line, when the tool is active. In this case, the distance is the height of the new rectangle.
Then, with snap on endpoints active, you can draw the left and right vertical lines using the 2-points line tool.
Or, if you need to have a closed rectangle, start with the parallel line too, but then use the rectangle tool with snap on endpoints active. Use the existing rectangle for the bottom left point and the right end of the parallel line as top right point of the new rectangle. At last, you can delete the parallel line again.
I´m planning furniture´s. I start with the corpus. Then I plan the wall´s and back wall… So, I have to too it very often. I know as well a lot of ways. Measure, Helping line, intercept point. But that not what I’m looking for. I look for a short way.
I’m looking for a way like auto cad or draw sight:
Rectangle - snap point left then right and then insert the height.
Rectangle - snap point left - then only X snap point right - and for Y insert value.
flywire - sorry what do you mean - tutorials or user manual - I haven´t found something but it is hard to use the search for my question in a foreign language
Yes, a foreign language is hard to do. The tutorials and user manual have some good exercises that would help do this. You could look at youtube videos too. They are in mostly in English and I don't know about your language.
In LibreCAD there is no such quick way to do it like in AutoCAD.
If you want to stick with LC it is better to accept working with the full procedure: Rectangle - snap point left - spacebar to activate command line - @x,y - Enter. Or LordofBikes advice with parallel. Or...
Maybe some day LibreCAD will have that quick way, maybe.
I have an idea, if you want to spare yourself to measure and type in the x value, you can set and lock relative zero to the right snap point, then for specify second rectangle corner type in the command line @0,your wanted y-value, enter. If you have done that once, set and lock relative zero are quickly at hand with right mouse click.
It would be better if relative zero could be set in between, during the rectangle operation (I think that's called a transparent command), without canceling it. You don't have to lock it then. Works that way in QCAD, tested. Room for improvement...