How to get list elements of jira custom field with c#?

I'm trying to get elements of custom fields from JIRA.I want to add these elements to windowsform combobox. I can get id and name of custom fileds but I need to get element of dropdonwlist. The customfields are project spesific.

I've been tried code below but it only takes name list of customfileds, but I need to get elements of dropdownlist from jira which is called Activity.

var jira = Jira.CreateRestClient(JiraUrl, txtBoxUsername.Text, txtBoxPassword.Text);   

var issue = jira.Issues.GetIssueAsync(cmBoxCRs.Text);

var customField = jira.Fields.GetCustomFieldsForProjectAsync("BKH").Result;

And I've already tried this:

cmBoxActivity.Text = issue.Result.CustomFields["Activity"].Values[0]

It only returned one value which has been selected on JIRA for spesific issue.