The first thing to do is to determine if the field that you are trying to generate identifier numbers for is a Ketryx field. Ketryx fields have a Jira field API id of the following format: com.ketryx.app.atlassian__<field>. For example the Ketryx field Requirement Type has the following Jira field API id: com.ketryx.app.atlassian__requirementType.

The next thing to do is to access the Jira API for field options to generate the identifier numbers for the Ketryx select options. To do so, you must have ADMIN access to your atlassian environment and search in the following url format in your web browser: https://<environment>.atlassian.net/rest/api/3/field/com.ketryx.app.atlassian__<field>/option?maxResults=50000. This will return a json blob containing up to 50000 options that are available for that Ketryx select option field under the "values" key. An example of this output can be found below:

{"self":"https://<environment>.atlassian.net/rest/api/3/field/com.ketryx.app.atlassian__requirementType/option?maxResults=50000&startAt=0","maxResults":50000,"startAt":0,"total":3,"isLast":true,"values":[{"id":1,"value":"User/Marketing","properties":{},"config":{"scope":{"projects":[],"projects2":[],"global":{"attributes":["notSelectable"]}},"attributes":["notSelectable"]}},{"id":2,"value":"Legal","properties":{},"config":{"scope":{"projects":[],"projects2":[],"global":{"attributes":["notSelectable"]}},"attributes":["notSelectable"]}},{"id":3,"value":"Regulatory","properties":{},"config":{"scope":{"projects":[],"projects2":[],"global":{}},"attributes":[]}}]}

Under the "values" list you can see the mappings for the requirement type Ketryx select option field, including the mapping from requirement type: "User/Market" to the identifier number "1".