IF STRINGKEY condition for constant string inside constant string

Questions
1

HI,
I have two string variables

BLOCK:ConstantString
LABEL:CONTACTS
value = @CONTACTS
=> VAR @CONTACTS1
ENDBLOCK

BLOCK:ConstantString
LABEL:messageCONTACTS
value = $“insert message here < CONTACTS1 >”
=> VAR @messageCONTACTS
ENDBLOCK

And then I have a POST request with interpolated mode

How I can POST data but only if @CONTACTS1 is not empty ? so it doesn’t end up with "insert message here "

2 
BLOCK:ConstantString
LABEL:CONTACTS
  value = @CONTACTS
  => VAR @CONTACTS1
ENDBLOCK


if (!string.IsNullOrWhiteSpace(CONTACTS1))
{
BLOCK:ConstantString
LABEL:messageCONTACTS
  value = $"insert message here < CONTACTS1 >"
  => VAR @messageCONTACTS
ENDBLOCK

    // Perform the POST here
}
else
{
    
    LOG "CONTACTS1 is empty. Skipping messageCONTACTS block and POST."
}