How to set custom user-agent that Google Analytics can read

You can manipulate the User Agent but you can't control how Google will interpret the resulting device/OS:

• The processing is done on the server side (Google) so there is no way of directly modifying that data (even when sending data via the measurement protocol).

• The processing details are not disclosed by Google so you won't know what the outcome of your experiments are until they're reported by Google Analytics (which due to the 24-48 hour data processing latency might make such experimentation tedious).

• Attempting to manipulate it might "break" your analytics: Google is vague about this, they just say: "Google has libraries to identify real user agents. Hand crafting your own agent could break at any time". 2 consequences I can think of: Google simply drops the traffic if it can't parse the User Agent OR marks it as bot/spider traffic (which will also be dropped if you have enabled the bot filtering option).

Although it's not mentioned in the documentation, I also suspect Google to rely on other data points, which could be:

• Screen resolution
• Java Support
• Flash version

I couldn't find more details on the topic, and I don't think you will find more details from Google explaining what they use to calculate browser/device because they don't want people messing with it (analogy: you won't find details about which data points are used for SEO, because they don't want people messing with it). The 4 dimensions I listed (User Agent, Screen resolution, Java Support, Flash version), are to my knowledge the only 4 that are device-specific from all GA collects (others are derived from them): https://developers.google.com/analytics/devguides/reporting/core/dimsmets#view=detail&group=platform_or_device

As in MAX's answer it's true, it's very difficult to manipulate the user-agent while keeping all the attributes, Like OS, and rendering engine etc...

At the sametime I still want to target my app users with a custom user-agent, and be able to separate traffic from this webview app.

What I did is this:

1- Setting the custom user-agent

Instead of replacing the whole user-agent with a custom one, I appended this to the user-agent [AppID/AppVersion], found great info from this blog: Webviews and User-Agent strings.

Now the user-agent looks something like this:

Mozilla/5.0 (Linux; Android 9; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/68.0.3440.91 Mobile Safari/537.36 [Custom App/1.0.1]

Check: Correct way to format user-agent string in an Android WebView App?

2- Setting a custom dimension in Google Analytics

Since Google Analytics will mark all browser value visits from this agent as Android Webview, I went to assign a custom dimension to be able to identify the custom user-agent sessions and create a separate view for it.

In the backend with PHP I set the value of the dimension based on the user-agent.

<script>
<?php
  if(strpos($_SERVER['HTTP_USER_AGENT'], 'Custom user agent here')!==false)
  {
     $customAgent_value = 'your agent';
  }
?>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-', {
  'custom_map': {'dimension1': 'custom_agent'}
  });
gtag('event', 'custom_agent_event', {'custom_agent': '<?= $customAgent_value;?>'}); 
</script>   

This is working fine for me now. I can target users from a specific webview app, and at the same time am able to separate the traffic from different webviews in Analytics.