to start with you have not fully defined how the main OSC is setup i would add
OSCCON1 = $62;
secondly you have not set a clock source for timer1, as it is it expects a clock signal on the t1clk pps pin you probably don't want that
T1CLK = 1; ClockSource FOSC/4; 534mS
is more like it
thirdly the chip has auto context save for interrupts so wsave etc are not needed or even useful

i don't have a 16f15345 chip on hand and my simulator has no model for them either so the code is untested

but try this


Code:
#CONFIG
    __config _CONFIG1, _FEXTOSC_OFF & _RSTOSC_HFINT32 & _CLKOUTEN_OFF & _CSWEN_OFF & _FCMEN_OFF
    __config _CONFIG2, _MCLRE_ON & _PWRTE_ON & _LPBOREN_OFF & _BOREN_ON & _BORV_LO & _ZCD_OFF & _PPS1WAY_OFF & _STVREN_ON
    __config _CONFIG3, _WDTCPS_WDTCPS_11 & _WDTE_OFF & _WDTCWS_WDTCWS_6 & _WDTCCS_HFINTOSC
    __config _CONFIG4, _BBSIZE_BB512 & _BBEN_OFF & _SAFEN_OFF & _WRTAPP_OFF & _WRTB_OFF & _WRTC_OFF & _WRTSAF_OFF & _LVP_OFF
    __config _CONFIG5, _CP_OFF
#ENDCONFIG
'           


OSCCON1 = $62;
OSCFRQ = %101                ' 16mhz
 DEFINE OSC 16  
' lcd setup
 DEFINE lcd_lines 4         
 DEFINE lcd_commandus 1500    
 DEFINE lcd_dataus 44
 DEFINE LCD_DREG PORTB        
 DEFINE LCD_DBIT 4            
 DEFINE LCD_RSREG PORTC       
 DEFINE LCD_RSBIT 7           
 DEFINE LCD_EREG PORTC       
 DEFINE LCD_EBIT 6            
'
'-------------------- I/O ------------------------------------------------------
' 
  TRISC.4=1            ' input TASTER
  TRISC.5=0            ' output LED1 
  ANSELA = 0     ' ADC off all
  ANSELB = 0   ' 
  ANSELC = 0   ' 
'
'*******************************************************************************
''************************* DT_INTS-14 *****************************************
'
INCLUDE "DT_INTS-14_16F15345-A.bas"     ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"                ' Include if using PBP interrupts
'
ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1_INT,  _ToggleLED1,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
'
    
T1CLK = 1; CS FOSC/4;   534mS




T1CON = $31                ; Prescaler = 8, TMR1ON
@ INT_ENABLE  TMR1_INT     ; enable Timer 1 interrupts
'


'*************************** PIN NAMES ***************************************** 
  TASTER     VAR portC.4       '  
  LED1       VAR portC.5   
'
' test program for PIC16F15345 with DT_INTS-14
' when button pressed, display "LED-blinky"
' TMR1 interupts toggle LED1
start:
      GOSUB clearLCD  
      LCDOUT $FE,$80,"  TEST 16F15345"
      LCDOUT $FE,$94,"   20250825"
      LCDOUT $FE,$D4,"        push button"   
'               
      WHILE TASTER = 1 : WEND             ' wait for pressed taster
      PAUSE  500
' 
      GOSUB clearLCD 
           LCDOUT $FE,$80,"   LED-blinky"                                                         
'      
Main:
      PAUSE 1
      GOTO Main                                    
'
'*******************************************************************************
'
clearLCD:
      LCDOUT $FE,$01   
          RETURN
'
'---------------------[TMR1 - interrupt handler]--------------------------------
ToggleLED1:
     TOGGLE LED1
@ INT_RETURN