Set status 'pending' on Guest Entry form
The "status" on an entry isn't explicitly set like that. It is determined based on other properties of the entry. You will need to set these other criteria for the entry to appear as "pending". 

An entry is 'pending' if it is enabled and has Post and Expiration Dates in the future.

More information on status' and the criteria can be found on the docs.

UPDATE: Depending on your use case, the following answer may be helpful: Editorial workflow / custom status