Ah, for some reason I thought you were on Jira Cloud!
On on-prem, do what I wrote, but then on that page, click the "Pencil" icon to edit your preferences. In the URL you should see something like:
The pencil icon is parallel to "Preferences" on the Profile screen. See it?
Hover above it, and you'll see your user ID in its URL
Another way to find your ID?
Hover above your name in Jira - if you've left a comment in an issue, for example, and you'll see a URL with your user ID. Example:
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