Concerning the Autocomplete widget source property, the documentation says:

a query string is added with a term field, which the server-side script should use for filtering the results. For example, if the source option is set to "https://example.com" and the user types foo, a GET request would be made tohttps://example.com?term=foo.

if the source already has parameters e.g.

source:"http://tx.fhir.org/r4/ValueSet/$expand&url=http%3A%2F%2Fterminology.hl7.org%2FValueSet%2Fv2-0006&_format=jquery",

The string appended is still ?term={x}. If there is already a ? in the source URL, &term={x} should be appended