This article shows some of common usages of JIRA rest apis.
Note: This article assumes you have a username and password for calling rest api of JIRA.
Get values of a custom field for a project
You know about a custom field, and want to get its possible values like a drop down having alot of options.
There is an API: createmeta, which gives you every possible field require to create JIRA issues.
Note: This API is pretty expensive in terms of data. It can give you complete data for all the projects. But, you might be interested in one or two projects.
curl -u "username:password" "https://jira_url/rest/api/2/issue/createmeta?projectKeys=XYZ"
Above api call will give you very limited result, which has only the issue types, and no expanded fields. But, we are interested in getting data of a custom field.
curl -u "username:password" "https://jira_url/rest/api/2/issue/createmeta?projectKeys=XYZ&expand=projects.issuetypes.fields"
Now, you are telling JIRA to return expanded fields, and now you will get values of all custom fields that appear while creating a JIRA issue in your project XYZ, and for each issuetype.