EasyMorph treats commas in numeric values as thousand separators and therefore ignores them. Therefore entering "1,2" as a text value of a parameter is not possible. It's a known issue.
As a workaround, use a delimiter other than the comma, and then replace it using a calculated expression using the replace() function. For instance:
{A} = "1|2"
{B} = replace( {A}, '|', ',') // The result is "1,2".
This workaround is only necessary when a parameter value is entered manually. When a parameter value is assigned in calls/iterations with a value of another parameter or a column, no workaround needed.
If you didn't know it, you can use the preview pane in "Web request" and "Iterate web request" actions. The preview pane shows the exact URL of the web request.
In the screenshot below you can see:
Using the replace() function to create a comma-separate list of integers as a parameter value