How to create/save Options of SelectCFType by Javascript

[S N]

Hey there :)

I am trying to create a plugin for Jira Server with the SDK, which is able to create single select custom fields, that fetches its option by a REST API call. I wrote the Javascript etc. and when I am creating the issue, the javascript fetches the options and puts them into the custom fields. My problem now is, that if I choose a fetched value and press the create button, the custom field is not saved (I already rewrote the option validation). I had a look at the SelectCFType class, but I was not able to find out which function I have to change/rewrite, such that I am able to save a customfield value that is not created as an option beforehand in Jira. I found this link https://jira-developer.atlassian.narkive.com/uwGm6kJp/problem-with-select-list-custom-field 1, where he/she probably had the same problems in the end, but did not write how they were solved.
Any help is appreciated, thanks in advance!
Best, Simon