Random String Inside of A JSON in Bash

I Want to insert A random String inside of a JSON in a Bash script I'm using to send _bulk od documents in the API of Elasticsearch:

SECONDS=0;
for i in {0..1000}
do
echo "Number: $i \n"

curl --location --request POST 'localhost:9000/_bulk' \
--header 'Content-Type: application/json' \
--data-raw '{"index": {"_index": "yotam-les", "_type": "_doc", "_id": "A975463f711"}}
{"Amount": "3", "Quantity": "2", "Id": "A975463f711", "Client_Store_sk": "1109"}
{"index": {"_index": "yotam-les", "_type": "_doc", "_id": "975463943"}}
{"Amount": "7", "Quantity": "2", "Id": "975463943", "Client_Store_sk": "1109"}

'

done
echo "\n took " $SECONDS "seconds"
exit 0

For example, in this bash script, I want to do something like

"_id": "<RANDOMLY_GENERATED_STRING>"

where "_id": "A975463f711" this part is. and of course, I wish that each iteration would randomly generate the ID itself.

in order to create a random string I know I can do this:

chars=abcd1234ABCD
echo -n "${chars:RANDOM%${#chars}:1}"

but how do Inject it inside the JSON?

to be more clear my problem is when I try to do this:

"_id": "${chars:RANDOM%${#chars}:1}"

because of the --data-raw '...' - it can only contain a raw string

Thanks for your help.