Posteo doesn’t support custom domain natively but theres a few workaround:
1- for incoming you use either your registrar free email forwarding or you move dns to cloudflare and use cloudflare email routing, route to your posteo native address. Theres also forwardemail.net for that. For outgoing, posteo do support sending from their native address on behalf of address on your custom domain. Might need to add spf records to not have the on behalf outgoing mail be treated as spam since the on behalf thingy is technically spoofing. If you use thunderbird, for outgoing you use the built-in identity feature.
2- use simplelogin for both incoming and outgoing.
3- use addy.io for both incoming and outgoing.
All 3 is a bit hackish, would have either the incoming only for #1 and both incoming and outgoing for #2 and #3 being handled by outside third party, not posteo directly so there might be privacy and/or security issue there.