If you are using Postman to pull this information verses with Python there are a couple things that you will want to do.
Get familiar with setting up your environmental settings. Once you set these you will not need to add this to your requests anymore.
In your example if you already have the organizationId and set it and your key, url in your environmental variables you should be able to run that GET command {{baseUrl}}//organizations/{{organizationId}}/networks
If you go to the response body in Postman you will see the output of all your networkIds.
After that you can go one step further and pull data by either exporting those to a csv or manually defining on as a variable.
Defining the {{networkId}} variable can be set at the get / put level in your Params table or in your environmental if you are only working on one network or you can utilize Runner inside Postman to iterate through a cvs file that contains your networkIds.
{{baseUrl}}/networks/{{networkId}}/appliance/vlans/
Your cvs will look like;
Column A
Row 1 networkid
Row 2 L_<your-networkId>
Row 3 L_<your-next-networkId>
Row 4 and so on
In the csv below I only have two networks in that list which were the only ones I needed to update.
You can open runner select your csv file and it will show you the number of lines it will iterate through you can click the preview button and see the list of your networkIds. These will become your variable {{networkId}}
Feel free to message me if this seems helpful or you have a question around it.