Create a new tab in the current window. The argument defines to which domain the tab belongs:

local wezterm = require 'wezterm'
local act = wezterm.act

return {
  keys = {
    -- Create a new tab in the same domain as the current pane.
    -- This is usually what you want.
    {key="t", mods="SHIFT|ALT", action=act.SpawnTab("CurrentPaneDomain")},
    -- Create a new tab in the default domain
    {key="t", mods="SHIFT|ALT", action=act.SpawnTab("DefaultDomain")},
    -- Create a tab in a named domain
    {key="t", mods="SHIFT|ALT", action=act.SpawnTab{DomainName="unix"}},