API search issues which contain a specific custom field dropdown value

Hi,

I'd like to create a API GET URL that allows me to return all issues that have a custom drop down field set to a certain value, something along the lines of

url = "https://xxx.atlassian.net/rest/api/3/search/jql=customfield_12345.id=54321"

The custom field when I look at each issue has the associated data

"customfield_12345": {
"id": "54321",
"self": "https://xxx.atlassian.net/rest/api/3/customFieldOption/54321",
"value": "This is the selected value string"
},

Obviously I am not querying correctly, but I have tried multiple versions and looked around for similar examples, but no joy.

Any help appreciated