The id is exposed in a few places. The two I usually go for are:
Make a REST call to Jira asking for the metadata for "create issue" in a project that uses the field and has it on the "create" screen
Go to Admin -> Issues -> Custom fields, and find your field. Hover the cursor over the edit or configure links on the right, and you should see a tooltip with a url in it. The url will contain id=##### - that's the id number you're looking for!
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