if we want to mandatory text field based on the condition, if multiselect value ex "abc" selected, so text field should be mandatory,
You can use the Required IF feature of sugarcrm.
Where you can define the formula for making a field required, also you can mention the multiselect field in conditions with IsInlist formula, like below:
Where test_c is the name of your multiselect field.
Hope it helps.
Senior Software Engineer.