How do you insert a custom function into an excel cell using vba?

I need to put a formula into a column if the column next to it has a value.

Application.Worksheets(ws).Range("AC"&LastRow).Formula = "=IF(AB2<>"",("ABC(")&(AB2)&(")ABC(")&(Y2)&(");"), "")"

although I could use some help with the syntax.

Basically the final formula should look like

= "some text" & Cell AB2 & "some text" & Y2 & "some text"

if AB2 has a value. In the code above I get a end of statement error in VBA which I am guessing is because of all the parentheses.

If anyone can help me with this I'd appreciate it.