STRUM_How to specify the chain number in a PDB file

Dear STRUM team,

Here is the job details:

I would like to predict a missense mutation Phe124Leu in the MT-ND5 subunit of human mitochondrial complex I

PDB ID: 5XTD (human mitochonrial complex I)

Chain number: l (small letter of the capital letter L)

Mutation: Phe124Leu

I uploaded the 5XTD.pdb file online. How can I tell STRUM to work on the chain l (subunit MT-ND5), rather than the whole giant enzyme complex?

Thank you very much