"the future" is a set phrase of collective noun. It means in the time ahead.
There is no "in future" but only "in future XXXX". The word "future" here is an adjective. for example: We will add a subscription form in future issues of the magazine.
Also, 'in future' can mean from now on, while 'in the future' means something that hasn't begun yet:
In future, we'll deal with similar situations differently.
In the future, people will holiday on the moon.
;-)