Wanted to add new custom field

i have two tables TPS Interchange and TFM Trinary. Primary key of TFM Trinary is foreing key of TPS Interchange which is Trinary_id . TFM Trinary contains key Column which has some text data like TESTCALL, TESTEMAIL

also created a record type TPS Interchange  and record type TFM Trinary . Now i want to create an custom field  like 


if   'recordType!TPS interchnage.Trinary_id = 15 then it showsTFM Trinary .key= TESTCALL
 if 'recordType!TPS interchnage.Trinary_id = 20 then it showsTFM Trinary .key= TESTEMAIL otherwise Default null

How to do this ?


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