Best strategy for making an OAuth1.0a signature

[scase]

We have to authenticate to a target which uses a oauth1.0. We have got the necessary consumer key, consumer secret, token, and token secret. The issue I'm having is now getting the hmac sha-1 signature properly generated. I have attempted doing it in a javascript policy

function generateSignature(key, text) { 
    print("key: " + key)
    var _sha1 = crypto.getSHA1();
    _sha1.update(key);
    _sha1.update(text);
    var js_result = _sha1.digest64();
    return encodeURIComponent(js_result);
}

I have also tried using an HMAC policy to generate the signature:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<HMAC name="HMAC1-GetSignature">
    <DisplayName>HMAC1-GetSignature</DisplayName>
    <Algorithm>SHA-1</Algorithm>
    <Message ref="variable_containing_message"/>
    <SecretKey ref="private.secretkey"/>
    <Output encoding='base64url'>hmac_value</Output>
</HMAC>

Neither way seems to generate a signature which is acceptable to the target nor which looks similar to what Postman is generating--and is successful. I have seen examples, but don't see any that show clearly how to do it within an Apigee proxy. Any helpful suggestions or code examples would be greatly appreciated.

[apickelsimer]

There is an example posted on another question that includes a sample proxy that acts as an OAuth 1.0a client. See https://www.googlecloudcommunity.com/gc/Apigee/is-there-any-way-call-Jira-Rest-APIs-OAuth-1-0a-from-... for more information. YMMV.