Try JSON.stringify() in the body like so:
fetch('https://truncated.ap-south-1.aws.cloud.dgraph.io/admin', {
method: 'POST',
headers: {
'Dg-Auth': DGRAPH_API_KEY,
'Content-Type': 'application/json'
},
body: JSON.stringify({
query: `your-query`,
variables: `{ if there are variables here... }`
})
});