Rocket.Chat/app/livechat/server/methods/setCustomField.js

import { Meteor } from 'meteor/meteor';

import { LivechatRooms, LivechatVisitors, LivechatCustomField } from '../../../models';

Meteor.methods({
	'livechat:setCustomField'(token, key, value, overwrite = true) {
		const customField = LivechatCustomField.findOneById(key);
		if (customField) {
			if (customField.scope === 'room') {
				return LivechatRooms.updateDataByToken(token, key, value, overwrite);
			}
			// Save in user
			return LivechatVisitors.updateLivechatDataByToken(token, key, value, overwrite);
		}

		return true;
	},
});