These results are always difficult to calculate, as presidential votes are not often summed by congressional district. Some states do make this data available, but Texas is not one of them. Thankfully, TX-15 is not as bad as some districts in its definition, so we can calculate this with a bit of effort.
TX-15 contains the whole of Brooks, Duval, Jim Hogg, Karnes, and Live Oak counties. Collating this data is straight-forward. It also covers most of Guadalupe county, excluding precincts 304, 305, 308, 402, 404, and 411, a large proportion of Hidalgo county - 173 of its 257 precincts, and precincts 209 and 414 of Wison county. Collating this data is more tricky.
Precinct data for Guadalupe county can be found here. Removing from the overall county total the totals from the six precincts outside the district, we get our total district vote in this county:- Trump: 39,257, Biden: 22,650.
Precinct data for Hidalgo county can be found (thankfully in XLS format) here. From this, we can extract the data for the 173 precincts within TX-15, giving us totals for this county of Trump: 61,840, Biden: 89,514.
Finally, in Wilson county, we run into a problem in that precinct-level results for this county have not been published. However, excluding this county, Biden has a lead of 4,728 votes over Trump which is unassailable given the population of the two TX-15 precincts in Wilson county. For example, in the House of Representative vote, the precincts gave 1,759 votes for the Republican candidate compared to 290 for the Democratic candidate.
If data for these two precincts in Wilson county become available, we will be able to calculate the precise final presidential vote totals in the district. We can conclude, however, that Biden won the presidential election in TX-15.
Update January 15th:
The Daily Kos' 'Pres-by-CD' project has now completed its work in Texas. Most of the calculations above were correct, however I missed three additional precincts allocated to TX-15 which threw off the totals somewhat - I've updated the information above and in the table below accordingly.
Presumably by writing to the Wilson County Election Administrator, they have also found totals for Wilson County.
Nevertheless, these new totals don't change the result; of the votes cast for President within TX-15, a majority, around 50.7%, were won by Biden.