per my understanding, you want to know how to get access_token value from a JSON object which comes from response of get token http request .
I did a simple demo for you :

The details of the post request :

This is a simple request to get access token in Azure. the response will be :
{
"token_type": "Bearer",
"expires_in": "3600",
"ext_expires_in": "3600",
"expires_on": "1557995356",
"not_before": "1557991456",
"resource": "<-resource name->",
"access_token": "<-value of access token->"
}
As you can see this logic app is triggered by http request and it will do a post request to get an JSON object which contains an access_token. And finally it will return the access_token value in response .
So the key here is how to config so that we can get access_token value from the JSON response of step2 .
Let's open logic app code view, find "response" =>"body" and modify its value as : "@body('HTTP').access_token"

So that you can get the certain param from your JSON response of previous http request :
