Blueprint

You need one empty blueprint in your inventory. I think you can get it in the blueprint bench or with console command

"item blueprint 1".

In create mode you have to choose a short key for the blueprint menu or make a blueprint bench in workbenches.

You first have to craft an empty blueprint in the crafting menu (you can find it in the crafting menu in the "Equipment" category). You can then use it to select an area (similar to how it worked in the Java version) by defining a start and end position (first click defines a start position, another click then defines the end position, and everything between it will be stored in the blueprint). Hold your right mouse button to save the blueprint, this will bring up a dialog where you can set a name and folder for it