How to set dynamic value to expires_in field in oauth token?

[#1]

I am validating a saml token using thumbprint and want to set the expiry value of oauth token as per the saml token that is being validated.

I was able to get the value in seconds, but the oauth policy is not accepting the vaiable in the expires_in field.

I tried to pass the variable in the below ways.

<ExpiresIn><Ref>saml_sessionLength</Ref></ExpiresIn>

<ExpiresIn>saml_sessionLength</ExpiresIn>

<ExpiresIn>{saml_sessionLength}</ExpiresIn>

<ExpiresIn ref="saml_sessionLength"/>

Error :

Error creating object

Error occurred while validation of bean OA_GenerateAccessTokenSAML.xml. Reason: - Schema validation failed. Cause : For input string: ''. Line number : 8. Column number : 14. File name : OA_GenerateAccessTokenSAML.xml..