To add custom headers to an HTTP request object, use the AddHeader() method. You can use this method multiple times to add multiple headers. For example:
oRequest = RequestBuilder:Build('GET', oURI)
           :AddHeader('MyCustomHeaderName','MyCustomHeaderValue')
           :AddHeader('MySecondHeader','MySecondHeaderValue')
           :Request.

Content type headers

While the RequestBuilder attempts to infer the content type of the request from the request body, you can set the Content Type header, as follows:
oRequest = RequestBuilder:Build('GET', oURI):ContentType('application/json').
To set an Accept header that specifies the content type to be returned by the server, select from the following methods:
oRequest = RequestBuilder:Build('GET', oURI):AcceptAll(). //Accepts all content types
oRequest = RequestBuilder:Build('GET', oURI):AcceptContentType(CHARACTER). //Specify a content type
oRequest = RequestBuilder:Build('GET', oURI):AcceptHtml(). //Accepts HTML content
oRequest = RequestBuilder:Build('GET', oURI):AcceptJson(). //Accepts Json content
oRequest = RequestBuilder:Build('GET', oURI):AcceptXml(). //Accepts XML content

API reference documentation

To view the complete list of methods that you can use on an HTTP request object, refer to the RequestBuilder documentation or see the OpenEdge.Net API Reference documentation.