@aurobind.ravichandra FYI I moved your question to the Developers & API public secton so others can participate.
The best thing to do is if you can post the code you tried which did not work.
Having said that, I need to mention that I tried and was never successful in adding a task with a custom field value via CURL (see Api & Custom Fields - #12 by Phil_Seeman for specifics).
I added an answer that should work for this case in the topic Phil posted Api & Custom Fields - #18 by Matt_Bramlage that ought to also cover this question (tl;dr: use square brackets like custom_fields[1234567890]=Value as if the ID were a key in an associative array). Thanks!