I was able to get this code working:

let Table1 = base.getTable('Table 1');


let Record1 = await input.recordAsync("Select the Record",Table1)


let Table2 = base.getTable('Table 2');


let Query2 = await Table2.selectRecordsAsync();


let Records2 = Query2.records;


let d = [ ]


Records2.forEach(c => d.push({id: c.id}));


await Table1.updateRecordAsync(Record1.id,{'Linked Record Field': d});


let check = true


await Table1.updateRecordAsync(Record1.id,{'Checkbox Field': check});


output.markdown('# Done')

let Table1 = base.getTable('Table 1');


let Record1 = await input.recordAsync("Select the Record",Table1)


let Table2 = base.getTable('Table 2');


let Query2 = await Table2.selectRecordsAsync();


let Records2 = Query2.records;


let d = [ ]


Records2.forEach(c => d.push({id: c.id}));


await Table1.updateRecordAsync(Record1.id,{'Linked Record Field': d});


let check = true


await Table1.updateRecordAsync(Record1.id,{'Checkbox Field': check});


output.markdown('# Done')

I am new to Airtables. Where do save and run this code please and which language is it? Thanks!

Glad you were able to get it figured out, @Michael_Andrew! You could also combine those two updateRecordAsync calls into one –

await Table1.updateRecordAsync(Record1, {


    'Linked Record Field': Query2.records.map(r => ({ id: r.id })),


    'Checkbox Field': true


});

Welcome to the forums @Peter_Vollawetscher!

This code is run in the scripting block, which can be found in the blocks gallery in your base. It’s currently free to use on all plans through September 2020! Scripts are written in JavaScript, and can interact with the data in your base – you can read more on the scripting docs site.

await Table1.updateRecordAsync(Record1, {


    'Linked Record Field': Query2.records.map(r => ({ id: r.id })),


    'Checkbox Field': true


});

Thanks. I know very little about programming, but can generally figure out from trial and error. I was able to work it out from other response so give rightful credit here: