Custom hosts for DNS MX records

Hi - I’m trying to connect Keela to a domain in our account, but they require an MX record with a custom host (specifically, “reply”.) So far I haven’t found a way to change the host portion of the MX records - it just defaults to the full domain. Is there a workaround, or do I need to use different nameservers and configure the DNS somewhere else?

Here’s what I’m trying to implement, if that helps:

https://support.keela.co/hc/en-us/articles/360048831292-Connect-Your-Domain-to-Keela-via-SendGrid-If-your-domain-is-hosted-by-Google