Where to enter University Entrance Exam in UCAS

Hi
I am applying to Bsc Maths and Econ at LSE and therefore am planning to take TMUA. I am also applying to Cambridge for BA Econ so will submit the application in September which is way before the TMUA. Can someone please guide me to where should I enter in my application to indicate that I will be taking TMUA in the coming months and thereafter send the scores?
PS: I am an international student