Can I say that a memo is from someone c/o me?
Or is that not correct?
In this case I’d use “per”.
I would say (I just made this up),
Yours truly,
/your signature here/
Aeris
On behalf of Bossman
Alternatively (common for things like an assistant signing for a supervisor),
Yours truly,
/your signature here with handwritten “for”/
Bossman
If you are doing this for work your company may have a policy for it.
“c/o” is for incoming, not outgoing.
When I used to sign things for other people, it was always
*Teacake
pp* Boring Boss
The bits in italics being handwritten. The pp stands for “per procurationem”, which basically means “on behalf of”. This may be old fashioned, I don’t know, but it’s what I would still do.