Here is an example:
if {@formula} like "*ABC*"
then true
else false
Another example
if {@formula} like "A?C"
then true
else false
In the first case, the asterisk is for any number of characters, in the second, the question mark indicates one and only one character. You can use fields or formulas, with all the attendant precautions.
You can also use Instr in a similar fashion.
if {@formula} like "*ABC*"
then true
else false
becomes
if instr({@formula},"ABC",1)>0
then true
else false
Instr returns a numeric location in the string. Hence the >0 indicates anywhere. If you want to know if it were the first you'd make it
if instr({@formula},"ABC",1)=1
then true
else false
No reason not to use Like if you prefer it.
And of course in all of these cases you can remove the true false, like this:
{@formula} like "*ABC*"
because this returns a true or false and the if-then is implied. I included them above for clarity's sake. And you may want to return something other than true/false such as a string or a number, or the result of another equation.