In the eTrading platform, you have a table that lists your positions.
There, click on the three dots in the header of the first column.
My interface is in German, but you are looking for a new column, somewhere under “Positions”, that says something like “Registered shares”.
This will add a column to your table, showing how many shares you have registered; if you click on any number, a new page is opened. There, you can simply use the “+” and “-” buttons to add or remove shares from the register.