if condition in convert expression

[amu]

In the join query instead of setting the join relationship link,need to use "convert to expression" option

Is it possible to use a function as
"
IF ([Patient] = 'available')
    THEN ([Prescriber].[PRESCRIBER_KEY] = [Patient].[Patient_Key])

    ELSE ([Provider].[Provider_key] = [Patient].[Patient_Key])
"

I am getting error like "syntax error near "=" "