In Strategy Developer, create an attribute qualification prompt, and set the display style to “Text Box” and Operator to “In list” or “Not in list”. The default prompt answer is set to for example “a;b”:

In previous versions of Strategy, such as version 10.4, when the prompt is executed in Strategy Web, “a;b” is treated as 2 elements.
After upgrading to Strategy One, March 2024, when the same prompt is executed in Strategy Web or Library, a backslash is added before the semicolon:

Strategy introduced a behavior change in Strategy 2019 Update 3.
To support multiple elements in a prompt, a semicolon is used as a delimiter when the prompt’s display style is set to “Text Box” and Operator is set to “In list” or “Not in list”. For example, if “a;b” is entered as the prompt answer, it will be treated as 2 elements: “a” and “b”.
Before Strategy 2019 Update 3, there was a limitation where if a value contained a semicolon, it was always treated as a delimiter and could not be part of the value.
To address this limitation, Strategy made a change in Strategy 2019 Update 3. Now, if "a;b" is entered as the prompt answer in Strategy Developer, it will be considered as one element by default. To escape this special character, a backslash is added before the semicolon (for example: a\;b) in the prompt page for Strategy Web and Library.

Check the prompt details and the prompt answer is “a;b”.
Users upgrading from versions before 2019 Update 3 must manually make the change in order to maintain the same behavior as in previous versions. Currently, there is no option to make bulk changes:
Run the prompt in Strategy Web or Library. Manually modify the value and delete “\” in the prompt page:

Check the prompt details. The prompt answer is changed to “a” and “b”:
