The template
Subject: For a future issue of {newsletter name}
Hi {first name},
I've been reading {newsletter name} for a while. The {recurring section, e.g. "Tools" or "Worth reading"} section is how I found {something they featured}.
Something that might fit a future issue: {what it is, in one line}, {why their readers would care}. {Proof: a number, a free plan, a launch date}.
Happy to send anything that makes it easy to include: a short blurb, a screenshot or a free account.
Thanks,
{your first name}