Ok, there’s something I’m not getting. You say you want to get the access token in your question’s title, but you also say you’re having trouble with oauth/request_token, which is not the endpoint to get the access token (the endpoint to get the access token is oauth/access_token).
If it really is the request token you’re having trouble with, then you shouldn’t be putting any oauth_token in both authorisation and signature, because you have not generated any tokens yet (that’s why you are “requesting” your first token with oauth/request_token).
So, under the assumption that you’re having problems with the request of the token, your code should look like this:
**Authorization = "OAuth oauth_callback=“twitterlistmydemo%3A%2F%2Fsuccess%2F”, oauth_consumer_key=“iFD43xOsYhovALC7PMPeGhK58”, oauth_nonce=“7537F128-D398-4494-8E95-982A4C96E82B”, oauth_signature=“VrAylhhdeKUqOzkOQLNX03vv92Y%3D”, oauth_signature_method=“HMAC-SHA1”, oauth_timestamp=“1551785440”, oauth_version=“1.0"”;**
…And the signature will be different too, because you’ve created yours using a token that doesn’t exist yet.
Best of luck!