As was suggested by @benn, you will need to add the TdTomato sequence to the end of your fasta formatted genome file. If your TdTomato sequence is in fasta format already,
cat will do the trick. If not, just go the end of the file, start a new line and add a header line starting with ">" then immediately the name of your gene, no whitespcaes in between, something like ">TdTomato". Then add your sequence in a new line.
The format of the GTF file is explained in the image you posted, the trick is to use the exact same gene name (in column 1) that is used in the genome file you will create at the previous step (no ">" though). Again, this line, in which the different fields (columns) will be separated by tabs, will be added to the end of your GTF file.