Custom Button - Placement no drop down list

Hello I am trying to follow the instruction here, but i cannot proceed because placement doesn’t have drop down list . please help me how to install it properly

Start typing in the placement box like “sales” and options will appear.

Thank you. problem resolved!

It does not work for me. Anyone please help

you can use:

sales-invoices

The button will appear at the bottom of your sales invoice tab.

No "