If you are using the MJML email template, to create custom social icons you need to follow these instructions: MJML Custom Social Element Guide.
However, I am not sure if you can edit the code directly in the builder and have it work, you will need to test it out.
Since you intend to use two custom social icons (TikTok and WhatsApp), please ensure that the images have the same height and width to maintain harmony in your theme.