I think I will get a bonus this month. (Using this month specifies exactly the month you are in). If you were to know that it would be in another month, then you would say.

I think I will get a bonus in December. (This is because the name of the month is mentioned and it is far off yet. Or another way to say it using the month’s name is:

I think I will get a bonus this December.(Here you specify that is this December comming up.)

You can also say the following:

I believe I will get a bonus this month.

Believe (because you are more sure about it and there was a mention of it)

I think (because you are not sure yet, you have not heard about it and you only suspect you will get one).

Hope this helped you a bit.

I think i will get a bonus this month.

there is no ” in” or anything else in between.