github's post pulls api throwing invalid head error for a forked head in same org

Question

repo1: https://github.com/ORG_SPACE/MAIN_REPO

repo2(forked from repo1 in same workspace): https://github.com/ORG_SPACE/FORKED_REPO

feature branch created in forked repo: https://github.com/ORG_SPACE/FORKED_REPO/tree/FEATURE_BRANCH

Authorization details: used new finegrained PAT having pull:read&write,contents:read,Metadata:read access of both repos MAIN_REPO,FORKED_REPO

ISSUE: getting 422 error response

{
    "message": "Validation Failed",
    "errors": [
        {
            "resource": "PullRequest",
            "field": "head",
            "code": "invalid"
        }
    ],
    "documentation_url": "https://docs.github.com/rest/reference/pulls#create-a-pull-request"
}

while hitting below curl to raise pull request to merge 'ORG_SPACE/FORKED_REPO/tree/FEATURE_BRANCH' into 'ORG_SPACE/MAIN_REPO/tree/master'

curl --location --request POST 'https://api.github.com/repos/ORG_SPACE/MAIN_REPO/pulls' \
--header 'Authorization: Bearer MY_PAT' \
--header 'Accept: application/vnd.github+json' \
--header 'Content-Type: application/json' \
--data-raw '{
    "title": "test",
    "head": "ORG_SPACE:FEATURE_BRANCH",
    "base": "master"
}'

• tried this head value : "ORG_SPACE:FORKED_REPO/FEATURE_BRANCH" but didn't work.
• raising pull request with same api working fine for both repos while head and base are in same repo.