You can use the google site specific tag to search only a specific forum or subforum by using the following string.
this searches only the Off-topic forum i.e. /forum/412 at the Stardock site i.e. forums.Stardock.com for the string "gop" and finds my thread "The GOP Plan" that's in the Off-topic forum.
To use this you do need to understand a bit about how Stardock's forum structure is laid out.
In your example I have no idea where the multiplicity forum is but if you can navigate to it the url in your browser will give you the forum or subforum number. For example I'm on the Stardock site and I navigate to the Home > Object Desktop > WindowBlinds subforum and my broswer points to https://forums.stardock.com/forum/167 which is the WindowBlinds subforum on the Stardock site.
This will be the similar on other Stardock sites that also have access to the WindowBlinds subforum. For example on the Wincustomize site the WindowBlinds subforum is accessible at the slightly different path of Home > Stardock Support > Object Desktop > WindowBlinds but the syntax to access that subforum from Wincustomize is the same https://forums.wincustomize.com/forum/167.
However not all forums are accessible from all Stardock sites and this syntax won't work on sites that have no access to the particular subforum.