This is a similar question

https://forum.ninox.com/t/p8hb6tq/set-choice-value

You could adapt the script to meet your needs

Why do you need a list of all choices in a multiple choice field?

Do you need it in a single field?

Since a multiple choice field is probably static and pretty short then you can just edit the field and write the selections down into a text document.

If the list is longer than 6 or 7 or is changing, then I would recommend that you switch over to a dynamic multi-choice field.

There may be other ways but this is one... First, this will not work in a formula field as we need a trigger. In this example i used a button.

Also we need a (multiline)text field, in this example called ResultButton. My multiple choice field is here 'M Choices'.

let t := numbers('M Choices');
'M Choices' := "fffff";
ResultButton := join(chosen('M Choices'), "
");
'M Choices' := t

After a push of the button we get all possible values of the multiple choice field. If you want to automate the trigger instead of a button, you can use maybe the trigger of a tab-element.

Steven