How to define an outgroup to build a robust amino acid tree

I building a robust amino acid phylogeny with adequate robustness support (see previous post here).

This is a brief summary of what I have done:

1. I performed a BLASTp analysis using a protein sequence from species A as query against the full NCBI protein database,
2. downloaded all the hits in fasta format and I added the sequences I identified with the query sequence in my target genome, which is closely related to species A.
3. I then aligned the sequences using the Compute – Alignment – MUSCLE option in Seqotron and I saved my alignment file in Phylip format.
4. I tried to run this command line in RAxML to build a robust maximum likelihood tree:

raxmlHPC -m PROTGAMMAILG -n output.tre -o outgroup-p 10000 -s ~/Desktop/alignment_file.phy

However, the program showed the following error:

Error, the outgroup name "outgroup-p" you specified can not be found in the alignment, exiting ….

Question how can I build an outgroup for my phylogenetic tree, and how do I include it in my alignment?

I appreciate your help and suggestions!

raxmlHPC -m PROTGAMMAILG -n output.tre -o outgroup -p 10000 -s /Users)/username/Desktop/alignment_file.phy