@Patrick_Unverricht you can pass header values from modeler like below
<camunda:inputParameter name="headers">
<camunda:map>
<camunda:entry key="Accept">application/json</camunda:entry>
<camunda:entry key="Content-Type">application/json</camunda:entry>
<camunda:entry key="Authorization">Basic syuiisg398092hds</camunda:entry>
</camunda:map>
</camunda:inputParameter>
Credentials should be Base64 encoded string. In Linux, this command would do that: echo “username:password” | base64
or give a try with Jsoup: