You can use Dynamic sql . the code below fetches all users related to a specific role and then grant permission impersonate on a user. You should create a user on application login to relate it with database , then grant permission to impersonate on all members of specific role. This is the code:
CREATE TRIGGER S2
ON DATABASE
FOR CREATE_USER
AS
CREATE TABLE #T
(PRINCIPAL_NAME NVARCHAR(100),ROLE_NAME NVARCHAR(100));
WITH L AS (SELECT *
FROM (SELECT P.name AS 'PRINCIPAL_NAME',R.role_principal_id AS 'GH'
FROM SYS.database_principals P,sys.database_role_members R
WHERE P.principal_id=R.member_principal_id OR P.principal_id=R.role_principal_id
AND type<>'R') S INNER JOIN (SELECT P.name AS 'ROLE_NAME',P.principal_id AS 'GHA'
FROM SYS.database_principals P,sys.database_role_members R
WHERE P.principal_id=R.member_principal_id OR P.principal_id=R.role_principal_id
AND type='R') D
ON D.GHA=S.GH)
INSERT INTO #T
SELECT DISTINCT PRINCIPAL_NAME,ROLE_NAME
FROM L
------------ ENTER ROLE NAME HERE
WHERE ROLE_NAME LIKE '%%'
------------
DECLARE @P NVARCHAR(100),@TEXT NVARCHAR(MAX)=''
------------------------- CHANGE IT TO YOUR DESIRED NAME OF YOUR APPLICATION USER
DECLARE @APPUSER NVARCHAR(100)='APPLICATION_USER'
-------------------------
DECLARE C CURSOR FOR SELECT PRINCIPAL_NAME FROM #T
OPEN C
FETCH NEXT FROM C INTO @P
WHILE(@@FETCH_STATUS=0)
BEGIN
SET @TEXT+='GRANT IMPERSONATE ON USER::['+@P+'] TO '+@APPUSER+' '
FETCH NEXT FROM C INTO @P
END
CLOSE C
DEALLOCATE C
DROP TABLE #T
EXEC(@TEXT)
I hope it work for You.