Emulating a key being pressed down continuously

Following images shows how to do this on the Switch Panel.
For Stream Deck the procedure is the same, you just use the "Key press" configuration for a certain button and use the options for "... Button Pressed" and add VK_NULL to "... Button Released".

VK_NULL, means [Virtual Key NULL] where null means "having no value". A VK_NULL doesn't produce a key press but it used to cancel other key presses being active.