Assist button for Dynamic Image source not showing up

Hi Community,

we have a scenario where we need to send a Marketing email to customers and the header image should be dynamic based on customer type and we went ahead with image block on new marketing email, and we did not see the Assist button.

https://docs.microsoft.com/en-us/dynamics365/marketing/create-marketing-email

I followed point no 9 from above msft docs to create it.

Though I am able to achieve above scenario using code element with if else statements but I guess the above approach is more efficient, need help on it.

Thanks