Elaborate on that.

I mean what button and where.

What type of custom parts?

Make them

A custom print, or an geniunely "custom" part that is not an official Lego mold part? For the custom decorations, see here:
https://www.mecabricks.com/en/forum/topic/2925

But is there a way to add existing parts that are not in the library?

@Theo0005 Not automatically. You can submit them via email for review. However bear in mind that the part shall be modelled as per the Mecabricks standards. Mesh exported from other platforms will not be accepted.

In what form should it be?

@Sealund, It would be nice if that was an update, it would be very useful!

can you like, 3d print these parts or something and get them in real life?

Why can you not make custom parts with meshes? I had some parts I wanted to add. ( I was bored in class and wanted to make a few 1 by √2 plates for diagonals, the models are at https://www.tinkercad.com/things/cZPkHRs8Eby-epic-kieran-esboo )