보고 싶어(요) (the 요/yo can be removed if you are close/friendly with the person)
bo-go si-po-(yo)
The structure VERB (base form) + 고 싶다 = I want to VERB, so adding 보다 (just the 보 part cos it's the base form of the verb) to 고 싶다 literally means "I want to see you", but it is most commonly used to mean "I miss you".
Below are some other ways to write/say it:
보고 싶다 bo-go sip-da
보 고 싶당 bo-go sip-dang (cute form)
보고 싶어용 bo-go si-po-yong (cute form)