Link up mailbox to exchange online from on-premise exchange

We got customer using the below.

1 x on-premise exchange server 

Active M365 admin portal with users account synced online via AD sync from on-premise AD but without mailbox.

Mailbox are on-premise exchange.

Now we need to assign M365 mailbox to active user account on exchange online.

Because we tried assign in exchange online but it prompt on-premise exchange server detected and can't proceed.

What is the best method to do so?

Thanks.