It's often called "generic button" or "middle button", but it may depend on the drivers.
Download this and drag it to the list in Preferences / Toolbars / Scripts:
Checkbox Mode Always.js.txt (533 Bytes)
Script code for reference (same as in the download):
function OnInit(initData)
{
initData.name = "Checkbox Mode Always";
initData.version = "1.0";
initData.copyright = "(c) 2020 Leo Davidson";
initData.url = "https://resource.dopus.com/t/checkboxes/34202";
initData.desc = "Turn on Checkbox Mode in all folders";
initData.default_enable = true;
initData.min_version = "12.0";
}
function OnAfterFolderChange(afterFolderChangeData)
{
var cmd = DOpus.Create.Command();
cmd.SetSourceTab(afterFolderChangeData.tab);
cmd.RunCommand("Set CHECKBOXMODE=On");
}