I am self-employed (actor). I have paid my agent commission. I have also paid her HST on top of that commission. Where do I claim both of these amounts on my T2125 form?

If you are registered to collect and remit GST/HST, you report your collections and input tax credits on a GST/HST remittance report, or online.  It is not reported or claimed on the T2125.

If you are not a registrant, you will include the GST/HST in all of your expenses.  You will not charge or remit GST.  In this way, you are not getting a refund of HST, but you are able to add it to the expense and deduct it.

Commission paid can be entered on line 8860 for professional fees or 9270 for "other expenses."