To enter the Marriage Allowance Transfer claim you will first need to amend your Personal Details to include your Spouse/Civil Partner details. Ensure you click 'Yes' to the box stating 'DO YOU INTEND TO CLAIM MARRIAGE (TRANSFER) ALLOWANCE OR MARRIED COUPLES ALLOWANCE (DOB PRE 06/04/1935)? IF SO PLEASE CHOOSE YES BELOW?' This is where you'd fill in your spouses personal details.
The response to the question results in HMRC adjusting the filed self-assessment return to reflect the transfer claim.
No change in the calculation of tax on your return is visible other than the response itself. When you return has been received by HMRC they will then validate the claim and adjust your tax calculation accordingly.
Please note: You must first make a claim via the HMRC website at https://www.gov.uk/apply-marriage-allowance