The T6 is an entry level camera model and the built-in flash does not have wireless "Master" capability.
BUT... If your external flash is equipped with slave modes S1 and S2, you should be able to get it to fire in sync with the flash on your camera. Use S2 so that it will ingnore the ETTL pre-flash from your cameras built-in flash.
You will need to set the flash power manually as this type of slave flash has no way of measuring, or automatically adjusting the light output required.
Which flash do you have?
Here is a sample of what S1 and S2 may look like in the flash menu.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM