Set custom fields on a Jira issue via REST API v3 (discover field IDs, value shapes, editmeta)

Documented steps

1. GET https://your-domain.atlassian.net/rest/api/3/field — lists all fields with id (customfield_XXXXX), name, custom flag, and schema.
2. GET /rest/api/3/issue/{issueIdOrKey}/editmeta — per-issue editable fields keyed by ID with allowedValues, required, operations (["set","add"]...), schema.custom (e.g. com.atlassian.jira.plugin.system.customfieldtypes:multiselect) and schema.customId.
3. Set values in fields by customfield key. Documented shapes: date picker "09/Jun/19"; datetime "06/Jul/19 3:25 PM"; multi-checkboxes ["10000","10002"] (option ids); single select {"value":"red"}; textarea = full ADF doc; single group picker "jira-software-users" (name string); multi-group ["jira-administrators","jira-software-users"].
4. Update an existing issue: PUT /rest/api/3/issue/{issueIdOrKey} with {"fields":{...}} for direct set, or the update map with per-field operation arrays (set/add/remove/edit/copy) to modify individual values.
5. Resolve valid select options via GET /rest/api/3/field/{fieldId}/context/{contextId}/option instead of guessing option text (options are context-scoped).
6. Auth: Basic (email:API token) or OAuth2 — read:jira-work for GET /field and editmeta, write:jira-work for PUT issue.
7. Official docs: https://developer.atlassian.com/cloud/jira/platform/rest/v3/api-group-issue-fields/