Inserting a BIMI-Selector header is not required, but it is recommended. It gives you more flexibility to define different logos for different brands. It also allows companies that only use SPF authentication to implement BIMI if the BIMI-Selector header is inserted into an email using the DKIM authentication signing mechanism.
If you don't insert a BIMI-Selector header, the BIMI lookup is performed on the "from:" (RFC5322) domain using the "default" selector. This means that you need to create a BIMI TXT record in the DNS for the "from:" domain that looks like this: