The store name it asks for is not your brand name
The store name the İkas connection asks for is not your brand's display name but the part that comes before myikas.com in your address. If your store is examplestore.myikas.com, only examplestore goes here. When it is wrong the connection fails and the error usually points at the credentials — while the real problem is the address. This is the single most common time sink during setup.
The client ID and client secret are generated in the İkas admin. Next to them sits an optional webhook token: fill it in and notifications from İkas arrive over a secret address; leave it empty and verification falls back to the merchant identifier inside the payload.