Ask for merge mode explicitly in Trigger Build dialog

Would it be possible to have “Trigger custom build” ask for mode along with configuration? It is extremely unobvious that “custom config” would be merged with existing configuration instead of replacing it. Or at least add some explanatory text with a link to that documentation page in the window.

Couldn’t agree more with the necessity of this: the current merge process (and lack of info about it in the UI) renders the custom build UI feature nearly unusable for me.