Add the badge to Ghost
Ghost has code injection built in at both site and post level. Site footer is the one you want: it applies to every post and page including ones written later, and unlike a theme edit it survives switching themes.
Where the code goes
Settings → Code injection → Site footer. Not the per-post code injection, which only applies to that one post. Site-level is what makes the badge appear everywhere.
Save, then reload a published post. Ghost applies injected code immediately, no restart, no rebuild. If you are behind a CDN, give it a cache cycle.
The code
<script src="https://maketaksh.com/badge.js" data-taksh="yourname" defer></script>Swap yourname for your handle. It is the one in your dashboard.
Editing the theme instead
You can add the line to default.hbs in your theme, and it works, but you lose it the moment you switch or update the theme, and Ghost themes update more often than most. Code injection is stored outside the theme, which is the whole point of it.
Checking it worked
Load the published page and look at the bottom corner. If nothing is there, it is almost always one of four things, in order of how often: the handle has no projects in it yet; the handle is wrong; the platform stripped the tag, as described above; or a content blocker ate it, check the Network tab for badge.js.
Once it is live, the Analytics panel in your dashboard lists every site the badge has been seen on, from the first page load, which is the fastest way to confirm an install on a site you cannot easily open.
Other setups
WordPress · Hugo, Jekyll and Eleventy · All twelve setups
What the badge is, and why cross-linking your own launches is worth doing at all: the short version.