Ok ... how did you miss the fact that they were withholding the wrong state taxes all year ?  Make sure to fix that asap if you have not yet done so.

Texas doesn't have an income tax  so you will need to file a non resident LA return to get the withholding  back. 

Why was your employer withholding LA tax, if you did not physically work in LA.  The only way to get it back is file a non resident LA return to claim a full refund.

Your question is "How do I apply it as Texas income?"

What is it you are trying to do?  In the state interview, you are usually asked to allocate your income as LA income or non LA income.  I'm not specifically familiar with the LA software, so you may not get an allocation screen for wage.  The alternate is this workaround: At the w-2 screen split the boxes 15-20 W-2 info into 2 lines. On the 1st line leave LA in box 15 but make box 16 blank or 0; leave the LA withholding in box 17. On the 2nd line put your home state  (TX)  abbreviation in box 15 and the state wages amount in box 16; leave box 17 blank or 0..