Add test (idaholab#23557)

framework/doc/content/source/mesh/MooseMesh.md

@@ -36,6 +36,7 @@ You could also use the [SideSetsFromNodeSetsGenerator.md] class to construct sid
 sets (or the [NodeSetsFromSideSetsGenerator.md] for vice versa). These object allows you to manually
 construct the relevant side sets and node sets earlier in the mesh generation process.
 
+To declare subdomain IDs which are not found on the mesh right after the mesh is setup, a list of IDs can be specified using the `Mesh/add_subdomain_ids` parameter. This permits setting up block restricted objects that will only act on subdomains created during the simulation (e.g. using the[CoupledVarThresholdElementSubdomainModifier.md]).
 
 ## Minimal interface
 

test/tests/mesh/add_subdomain_ids/add_subdomain_ids.i

@@ -0,0 +1,56 @@
+[Mesh]
+  [generate]
+    type = GeneratedMeshGenerator
+    dim = 2
+    nx = 10
+    ny = 10
+  []
+  add_subdomain_ids = 999
+[]
+
+[Variables]
+  [u]
+  []
+[]
+
+[Kernels]
+  [diff]
+    type = Diffusion
+    variable = u
+  []
+  [diff2]
+    type = MatDiffusion
+    diffusivity = 2
+    variable = u
+    block = 999
+  []
+[]
+
+[BCs]
+  [left]
+    type = DirichletBC
+    variable = u
+    boundary = left
+    value = 0
+  []
+  [right]
+    type = DirichletBC
+    variable = u
+    boundary = right
+    value = 1
+  []
+[]
+
+[Executioner]
+  type = Steady
+
+  # Preconditioned JFNK (default)
+  solve_type = 'PJFNK'
+
+  petsc_options_iname = '-pc_type -pc_hypre_type'
+  petsc_options_value = 'hypre boomeramg'
+[]
+
+[Outputs]
+  exodus = true
+[]

test/tests/mesh/add_subdomain_ids/gold/add_subdomain_ids_out.e

@@ -0,0 +1 @@
+../../splitting/gold/simple_diffusion_out.e

test/tests/mesh/add_subdomain_ids/tests

@@ -0,0 +1,10 @@
+[Tests]
+  [add_subdomain_ids]
+    type = Exodiff
+    input = add_subdomain_ids.i
+    exodiff = add_subdomain_ids_out.e
+    issues = '#23557'
+    design = MooseMesh.md
+    requirement = 'The system shall allow specifying valid subdomain IDs that do not exist on the initial mesh'
+  []
+[]