make google sheet as databae to send and get data depending on the parameter. need google app script expert

now I want to send data which is id and value and if the id exists add the data beside the old data with this symbol between them(---).
I want to use ajax call to end and receive data.
and if I receive data it will be only of the id send with parameter
the value is big text about 20k so it will besent with ajax and receive with it
I will use ajax from my website so it hould enable cord.