K (ngn/k), 13 12 bytes

-1 byte from @ngn's savings

{x,/y,'x}"!"

Try it online!

Creates a function projection, with x fixed to "!".

• y,'x pair-wise append an exclamation mark to each character in the input
• x,/ flatten the list, using "!" as the seed (to prepend the leading "!")

K (ngn/k), 13 12 bytes

-1 byte from @ngn's savings

{x,/y,'x}"!"

Try it online!

Creates a function projection, with x fixed to "!".

• y,'x pair-wise append an exclamation mark to each character in the input
• x,/ flatten the list, using "!" as the seed (to prepend the leading "!")

K (ngn/k), 13 bytes

{"!",/x,'"!"}

Try it online!

• x,'"!" pair-wise append an exclamation mark to each character in the input
• "!",/ flatten the list, using "!" as the seed (to prepend the leading "!")

K (ngn/k), 13 12 bytes

-1 byte from @ngn's savings

{x,/y,'x}"!"

Try it online!

Creates a function projection, with x fixed to "!".

• y,'x pair-wise append an exclamation mark to each character in the input
• x,/ flatten the list, using "!" as the seed (to prepend the leading "!")