According to the official example, your query should be of this kind:
curl --header 'Accept: application/sparql-results+xml' \
--data "query=SELECT+*+{?s+?p+?o.}" \
--user s472kd733007:bhrfk1aa8o0qlj7 \
'https://rdf.s4.ontotext.com/4032537848/wikidata/repositories/fast'
However, the query above does not work for me, whereas the query below does:
curl --header 'Accept: application/sparql-results+xml' \
--data "query=SELECT+*+{?s+?p+?o.}" \
--user s472kd733007:bhrfk1aa8o0qlj7 \
'http://awseb-e-m-awsebloa-11laimnu18r2i-2106042490.eu-west-1.elb.amazonaws.com/4032537848/wikidata/repositories/fast'
In the queries above:
s472kd733007
— API key,
bhrfk1aa8o0qlj7
— API key secret,
4032537848
— user id
wikidata
— database name,
fast
— repository id,
http://awseb-e-m-awsebloa-...eu-west-1.elb.amazonaws.com
— AWS instance address.
Visit tabs in your dashboard in order to obtain applicable values of these parameters.

As for update queries, see e. g. this answer. Your query should be:
curl --header 'Accept: application/sparql-results+xml' \
--data "update=INSERT+DATA+{owl:Nothing+owl:Nothing+owl:Nothing}" \
--user s472kd733007:bhrfk1aa8o0qlj7 \
'http://awseb-e-m-awsebloa-11laimnu18r2i-2106042490.eu-west-1.elb.amazonaws.com/4032537848/wikidata/repositories/fast/statements'
Please note that endpoint address in this request is different.