You can use the If window action to check if the Chrome window is open. Preferably using "By title and/or class" as the way to check the window and then you can use wildcards in the Window title and class. This might be needed, because the Chrome title will contain the web page title. So, you might leave it as it is, if you only want to attach to the browser that is already on a certain web page, or you might want to adjust it so that it finds any instance of Chrome. For example, the following settings would find any Chrome window:

You can then use the Launch new Chrome action, but choose to attach to a running instance, instead of launching a new one:

You can then attach to it by either using a URL, the title of the page or simply to the foreground Chrome (the latter will work if only one Chrome tab is open). You will then be able to work with this the same way as if it was launched as a new instance.
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