There are problems regarding Thai input to FM form fields that Thai vowel and tone-mark characters by default are not allowed to key in. So, I wrote this script, tied it to the trigger "OnLayoutKeystroke" and it works just perfectly. Hope this helps you folks for the Thai input requirements.
Thanks to CamelCase201507 for the guiding ( Re: Thai Language Support)
Ref. Thai Character Unicode - https://www.nectec.or.th/it-standards/thaistd.pdf