SPF records for workspace or domain email

[kolb]

how to hide the 'mailed by' address for SPF from the secondary domain which shows the primary domain name.
I am an admin of a workspace. I have added a secondary domain and then set up an email alias for the secondary domain. Then in the Gmail settings under account -> send mail as-> I have added the email alias of the secondary domain. Now when I send mail from that secondary domain email address, it displays the following thing to the recipient:

Mailed by: primary domain name
Signed by: secondary domain name

I want to either show (when mailed from a secondary domain email address) :

Mailed by: secondary domain name or hide it (and only show signed by: secondary domain name)