Which is correct? reason for why~ vs reason as to why~
Which is correct?

That's the reason for why I applied to this company.
That's the reason as to why I applied to this company.Is this also possible? That's the reason for I applied to this company.

Question

Which is correct? reason for why~ vs reason as to why~
Which is correct?

That's the reason for why I applied to this company.
That's the reason as to why I applied to this company.Is this also possible? That's the reason for I applied to this company.

Answer

"This / that is the reason why..." is correct.

Less formal - just "that's why"

Don't use contractions e.g. "that's" in formal writing.