Ref. No. 1361/42-759
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
‘Rajaba’ (with the Fat;ha of Baa) is correct. You should recite: Allahumma baarik lana fi Rajaba wa Sha’bana wa balligh-na Ramadan.
And Allah knows best
Darul Uloom Waqf Deoband