To generate an API key and link your Zotero account to Writer : 

1. Install the Zotero extension by following the instructions in this guide.
   
2. In the Writer editor, click on the hamburger menu in the top left corner, select the Extensions tab and click on the Zotero extension. You will have to enter an API key and User ID generated using your Zotero account here for just the first time accessing the extension, to link it to this extension.
   
3. To obtain your API key and User ID, log in to your Zotero account, go to Settings > Feeds API and click on Create new private key.
   
4. Copy the generated secret key, paste it in the API Key field in the Zotero extension page in Writer, and click Access. All citations and references from your Zotero repository will be listed in Writer.

To fetch and add citations to your Writer document:

1. Search for citations using one or more tags (using commas) or browse through the list in the library.
2. Pick the necessary citation(s) and click Add to Document Citations. These citations are added to the Citations & Bibliography panel.
   
3. Cite the sources in your Writer document from the Citations & Bibliography panel wherever required.

To delete the secret key from the extension  

1. Open a Writer document, click the hamburger icon, go to the Extensions tab, and select the Zotero extension.
   
2. Click the settings icon beside the Zotero extension's close icon, and click the Delete button. This will clear your secret key from the extension.
   

Note: You can also hide or uninstall the extension completely whenever required using these steps.