Airtable custom extension - programatically link "button" to the extension

[bogdanmax]

Hello,

I have successfully created new table from a custom extension. 

Nevertheless, I want to create "Button" field in that table that will invoke my custom extension.

There is an example in documentation which says I must obtain the URL of the block here:

{
    // The label of the button
    label: string,
    // URL the button opens, or URL of the block that the button opens.
    // Null when the URL formula has become invalid.
    url: string | null,
}

 What I am obviously missing is how to get URL of my block

Thanks in advance.