Atlassian.SDK update jira CustomField value on existing issues

[Christoph Roth]

Hello,

how do I update a CustomField value on a jira Issue using the API and C#? 

receiving the issue works fine, but 

myIssue.CustomFields["aaa"] = "asdf";

throws an error, telling me "Property or indexer cannot be assigned to -- it is read only"

Thanks in advance!