There are many congratulatory words and best wishes for wedding in Thai like in other languages. Mostly, you will have the words ขอ/ขอให้/ขออวยพรให้ begin in your blessing which it does mean "I wish/may (you)..." in English or "祝（你们）……" in Chinese. This question has been asked beforehand: https://www.italki.com/question/23790 Simply samples for Blessings and Wishes: http://www.thai-language.com/id/590127 http://en.bab.la/phrases/personal/best-wishes/english-thai/ In advance, just try google with keyword "อวยพรวันแต่งงาน", you will get the results with and without English meaning of Marriage Blessing Words in Thai, and here is one I've picked up from the list: https://goo.gl/1Tf2i3 P.S. However, since you aren't Thai people, just using English will be OK and you can express any word from your deep heart then.

ขอโทษที่ไม่ได้ไปร่วมงานนะครับ ขอให้มีความสุข รักกันนานๆ และตลอดไป