Thanks to community member @Chicken for being the first to devise a workaround for BlueMap's immutable UI!
Download or copy the BlueMapCustomSidebarButton.js file to your webapp, and register it. (guide)
You can customise the text of the button by changing the function parameters:
addLine();
createButton("https://bluemap.bluecolored.de/", "Visit BlueMap Website");
createButton("https://www.youtube.com/watch?v=dQw4w9WgXcQ", "Mischievous Button", true);The first parameter of the function createButton() is the URL the button will open when clicked,
the second is the text displayed on the button,
and the third is an optional boolean to make it open the link in a new tab.
You can have as many buttons as you want, just call the function createButton() multiple times.
You can place a line in-between the buttons by calling the function addLine().