How can spotfire add a calculated column in data table with lookup function

Question

I want to add a calculated column in my data table, but I can not find the function lookup function in expression function.

Layer ID	OVLTR	MT	Result
Cell A	NULL	MT_1
Cell B	Cell A	MT_2
Cell C	Cell A	MT_3
Cell D	Cell C	MT_4

Here is the example table, as you can see Layer ID and OVLTR have identical cell value but in different row. What I want to do is to find the matched Layer ID to OVLTR, and return MT of Layer ID.

Here is the result :

Layer ID	OVLTR	MT	Result
Cell A	NULL	MT_1
Cell B	Cell A	MT_2	MT_1
Cell C	Cell A	MT_3	MT_1
Cell D	Cell C	MT_4	MT_3

If you are familiar to excel, it is just a simple vlookup function, but I don’t know how to do it in spotfire?

I had done some search, some people say use and origin table and match the lookup table, this answer is no to me, because the origin table is 1GB excel and keep growing, it is difficult to me to use this method.

I also want to create custom expression function, but I don't know how, could someone please help me~~~~ no matter use the original function or custom function,

Thank You!!!!