We can complete the sentence as follows; "To apply you just have to fill out the form and send it in."
How to complete the sentence
Completing sentences in most cases would require using the correct prepositions. In the sentence provided, the most suitable preposition is 'out'. Thus, we can say; "To apply you just have to fill out the form and send it in."
The meaning of this sentence is that you have to complete your details in the job application and attach any necessary documents before sending them to the right recipients. So, the above is the correct form of the sentence.